linux-headers: fix circular dependency when HEADERS_AS_KERNEL is used

The linux-headers -> linux dependency causes a circular dependency, breaking
the source/legal-info/graph-depends/..  targets:

make graph-depends
Getting targets
Getting dependencies for ['toolchain-buildroot', 'toolchain', 'busybox',
'glibc', 'initscripts', 'linux-headers', 'skeleton', 'linux',
'host-fakeroot', 'host-makedevs', 'rootfs-cpio', 'rootfs-initramfs']
Getting dependencies for ['host-kmod', 'host-gcc-final',
'host-gcc-initial', 'host-gawk']
Getting dependencies for ['host-gmp', 'host-binutils', 'host-pkgconf',
'host-mpfr', 'host-mpc']
Getting dependencies for ['host-m4']

Recursion detected for  : toolchain
which is a dependency of: linux
which is a dependency of: linux-headers
which is a dependency of: glibc
which is a dependency of: host-gcc-final
which is a dependency of: toolchain-buildroot
which is a dependency of: toolchain
Makefile:721: recipe for target 'graph-depends' failed
make: *** [graph-depends] Error 1

Fix it by instead duplicating in linux-headers the 10-20 lines of linux.mk
logic that infer the _SOURCE/_SITE/_VERSION from the BR2_LINUX_KERNEL_*
variables.

This does mean that we extract the kernel sources twice though.

[Peter: use same git/hg tarball as linux kernel to not clone twice, minor fixes]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/linux-headers/linux-headers.mk

@@ -9,14 +9,67 @@
 
 ifeq ($(BR2_KERNEL_HEADERS_AS_KERNEL),y)
 
-LINUX_HEADERS_VERSION = none
-LINUX_HEADERS_SOURCE =
+LINUX_HEADERS_VERSION = $(call qstrip,$(BR2_LINUX_KERNEL_VERSION))
+
+# Compute LINUX_HEADERS_SOURCE and LINUX_HEADERS_SITE from the configuration
+ifeq ($(BR2_LINUX_KERNEL_CUSTOM_TARBALL),y)
+LINUX_HEADERS_TARBALL = $(call qstrip,$(BR2_LINUX_KERNEL_CUSTOM_TARBALL_LOCATION))
+LINUX_HEADERS_SITE = $(patsubst %/,%,$(dir $(LINUX_HEADERS_TARBALL)))
+LINUX_HEADERS_SOURCE = $(notdir $(LINUX_HEADERS_TARBALL))
+BR_NO_CHECK_HASH_FOR += $(LINUX_HEADERS_SOURCE)
+else ifeq ($(BR2_LINUX_KERNEL_CUSTOM_LOCAL),y)
+LINUX_HEADERS_SITE = $(call qstrip,$(BR2_LINUX_KERNEL_CUSTOM_LOCAL_PATH))
+LINUX_HEADERS_SITE_METHOD = local
+else ifeq ($(BR2_LINUX_KERNEL_CUSTOM_GIT),y)
+LINUX_HEADERS_SITE = $(call qstrip,$(BR2_LINUX_KERNEL_CUSTOM_REPO_URL))
+LINUX_HEADERS_SITE_METHOD = git
+# use same git tarball as linux kernel
+LINUX_HEADERS_SOURCE = linux-$(LINUX_HEADERS_VERSION).tar.gz
+else ifeq ($(BR2_LINUX_KERNEL_CUSTOM_HG),y)
+LINUX_HEADERS_SITE = $(call qstrip,$(BR2_LINUX_KERNEL_CUSTOM_REPO_URL))
+LINUX_HEADERS_SITE_METHOD = hg
+# use same hg tarball as linux kernel
+LINUX_HEADERS_SOURCE = linux-$(LINUX_HEADERS_VERSION).tar.gz
+else
+LINUX_HEADERS_SOURCE = linux-$(LINUX_HEADERS_VERSION).tar.xz
+ifeq ($(BR2_LINUX_KERNEL_CUSTOM_VERSION),y)
+BR_NO_CHECK_HASH_FOR += $(LINUX_HEADERS_SOURCE)
+endif
+# In X.Y.Z, get X and Y. We replace dots and dashes by spaces in order
+# to use the $(word) function. We support versions such as 4.0, 3.1,
+# 2.6.32, 2.6.32-rc1, 3.0-rc6, etc.
+ifeq ($(findstring x2.6.,x$(LINUX_HEADERS_VERSION)),x2.6.)
+LINUX_HEADERS_SITE = $(BR2_KERNEL_MIRROR)/linux/kernel/v2.6
+else ifeq ($(findstring x3.,x$(LINUX_HEADERS_VERSION)),x3.)
+LINUX_HEADERS_SITE = $(BR2_KERNEL_MIRROR)/linux/kernel/v3.x
+else ifeq ($(findstring x4.,x$(LINUX_HEADERS_VERSION)),x4.)
+LINUX_HEADERS_SITE = $(BR2_KERNEL_MIRROR)/linux/kernel/v4.x
+endif
+# release candidates are in testing/ subdir
+ifneq ($(findstring -rc,$(LINUX_HEADERS_VERSION)),)
+LINUX_HEADERS_SITE := $(LINUX_HEADERS_SITE)/testing
+endif # -rc
+endif
 
-LINUX_HEADERS_LICENSE = $(LINUX_LICENSE)
-LINUX_HEADERS_LICENSE_FILES = $(LINUX_LICENSE_FILES)
+LINUX_HEADERS_PATCHES = $(call qstrip,$(BR2_LINUX_KERNEL_PATCH))
+
+# We rely on the generic package infrastructure to download and apply
+# remote patches (downloaded from ftp, http or https). For local
+# patches, we can't rely on that infrastructure, because there might
+# be directories in the patch list (unlike for other packages).
+LINUX_HEADERS_PATCH = $(filter ftp://% http://% https://%,$(LINUX_HEADERS_PATCHES))
+
+define LINUX_HEADERS_APPLY_LOCAL_PATCHES
+	for p in $(filter-out ftp://% http://% https://%,$(LINUX_HEADERS_PATCHES)) ; do \
+		if test -d $$p ; then \
+			$(APPLY_PATCHES) $(@D) $$p \*.patch || exit 1 ; \
+		else \
+			$(APPLY_PATCHES) $(@D) `dirname $$p` `basename $$p` || exit 1; \
+		fi \
+	done
+endef
 
-LINUX_HEADERS_PATCH_DEPENDENCIES = linux
-LINUX_HEADERS_REAL_DIR = $(LINUX_DIR)
+LINUX_HEADERS_POST_PATCH_HOOKS += LINUX_HEADERS_APPLY_LOCAL_PATCHES
 
 else # ! BR2_KERNEL_HEADERS_AS_KERNEL
 
@@ -30,13 +83,11 @@ LINUX_HEADERS_SITE = $(BR2_KERNEL_MIRROR)/linux/kernel/v4.x
 endif
 LINUX_HEADERS_SOURCE = linux-$(LINUX_HEADERS_VERSION).tar.xz
 
+endif # ! BR2_KERNEL_HEADERS_AS_KERNEL
+
 LINUX_HEADERS_LICENSE = GPLv2
 LINUX_HEADERS_LICENSE_FILES = COPYING
 
-LINUX_HEADERS_REAL_DIR = $(@D)
-
-endif # ! BR2_KERNEL_HEADERS_AS_KERNEL
-
 LINUX_HEADERS_INSTALL_STAGING = YES
 
 # linux-headers is part of the toolchain so disable the toolchain dependency
@@ -53,7 +104,7 @@ LINUX_HEADERS_ADD_TOOLCHAIN_DEPENDENCY = NO
 # uClibc building. This way uClibc doesn't modify linux headers on installation
 # of "its" headers
 define LINUX_HEADERS_CONFIGURE_CMDS
-	(cd $(LINUX_HEADERS_REAL_DIR); \
+	(cd $(@D); \
 		$(TARGET_MAKE_ENV) $(MAKE) \
 			ARCH=$(KERNEL_ARCH) \
 			HOSTCC="$(HOSTCC)" \
@@ -64,7 +115,7 @@ define LINUX_HEADERS_CONFIGURE_CMDS
 endef
 
 define LINUX_HEADERS_INSTALL_STAGING_CMDS
-	(cd $(LINUX_HEADERS_REAL_DIR); \
+	(cd $(@D); \
 		$(TARGET_MAKE_ENV) $(MAKE) \
 			ARCH=$(KERNEL_ARCH) \
 			HOSTCC="$(HOSTCC)" \