fs: introduce per-rootfs TARGET_DIR variable

... which for now still points to the base target directory, but this is
a step forward.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

Makefile

@@ -215,8 +215,10 @@ BR_GRAPH_OUT := $(or $(BR2_GRAPH_OUT),pdf)
 
 BUILD_DIR := $(BASE_DIR)/build
 BINARIES_DIR := $(BASE_DIR)/images
+# The target directory is common to all packages,
+# but there is one that is specific to each filesystem.
 BASE_TARGET_DIR := $(BASE_DIR)/target
-TARGET_DIR = $(BASE_TARGET_DIR)
+TARGET_DIR = $(if $(ROOTFS),$(ROOTFS_$(ROOTFS)_TARGET_DIR),$(BASE_TARGET_DIR))
 # initial definition so that 'make clean' works for most users, even without
 # .config. HOST_DIR will be overwritten later when .config is included.
 HOST_DIR := $(BASE_DIR)/host

fs/common.mk

@@ -39,6 +39,7 @@ ROOTFS_USERS_TABLES = $(call qstrip,$(BR2_ROOTFS_USERS_TABLES))
 define inner-rootfs
 
 ROOTFS_$(2)_DIR = $$(FS_DIR)/$(1)
+ROOTFS_$(2)_TARGET_DIR = $$(BASE_TARGET_DIR)
 
 # extra deps
 ROOTFS_$(2)_DEPENDENCIES += host-fakeroot host-makedevs \