core: allow a br2-external tree to override a defconfig

Currently, it is not possible for a br2-external tree to override a
defconfig bundled in Buildroot, nor is it possible to override one from
a previous br2-external tree in the stack.

However, it is interesting that a latter br2-external tree be able to
override a defconfig:

  - the ones bundled in Buildroot are minimalist, and almost always
    build a toolchain, so a br2-external tree may want to provide a
    "better" defconfig (better, in the sense "suited for the project");

  - similarly for a defconfig from a previous br2-external tree.

But we can't do that, as the rules for the defconfigs are generated in
the order the br2-external trees are specified, all after the bundled
defconfigs. Those rule are patten-matching rules, which means that the
first one to match is used, and the following ones are ignored.

Add a new utility macro, 'reverse', inspired from GMSL, that does what
it says: reverse a list of words.

Use that macro to reverse the list of br2-external trees, so that the
latters win over the formers, and even over bundled ones.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Samuel Martin <s.martin49@gmail.com>
Cc: Romain Naour <romain.naour@openwide.fr>
Cc: Julien CORJON <corjon.j@ecagroup.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

Makefile

@@ -848,7 +848,7 @@ define percent_defconfig
 	@$$(COMMON_CONFIG_ENV) BR2_DEFCONFIG=$(1)/configs/$$@ \
 		$$< --defconfig=$(1)/configs/$$@ $$(CONFIG_CONFIG_IN)
 endef
-$(eval $(foreach d,$(TOPDIR) $(BR2_EXTERNAL_DIRS),$(call percent_defconfig,$(d))$(sep)))
+$(eval $(foreach d,$(call reverse,$(TOPDIR) $(BR2_EXTERNAL_DIRS)),$(call percent_defconfig,$(d))$(sep)))
 
 savedefconfig: $(BUILD_DIR)/buildroot-config/conf prepare-kconfig
 	@$(COMMON_CONFIG_ENV) $< \