package/linux: don't enforce check for DTS when not building

Currently, this is triggering the error message:
    make randconfig
    make source

Limit the checks that enforce a DTS is set and at most one DTB is
appended to when we are actually building, like is done for the
configuration-file variables.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

linux/linux.mk

@@ -88,6 +88,8 @@ else ifeq ($(BR2_LINUX_KERNEL_USE_CUSTOM_DTS),y)
 KERNEL_DTS_NAME = $(basename $(filter %.dts,$(notdir $(call qstrip,$(BR2_LINUX_KERNEL_CUSTOM_DTS_PATH)))))
 endif
 
+ifeq ($(BR_BUILDING),y)
+
 ifeq ($(BR2_LINUX_KERNEL_DTS_SUPPORT)$(KERNEL_DTS_NAME),y)
 $(error No kernel device tree source specified, check your \
 BR2_LINUX_KERNEL_USE_INTREE_DTS / BR2_LINUX_KERNEL_USE_CUSTOM_DTS settings)
@@ -100,6 +102,8 @@ $(error Kernel with appended device tree needs exactly one DTS source. \
 endif
 endif
 
+endif # BR_BUILDING
+
 KERNEL_DTBS = $(addsuffix .dtb,$(KERNEL_DTS_NAME))
 
 ifeq ($(BR2_LINUX_KERNEL_IMAGE_TARGET_CUSTOM),y)