[Buildroot] [PATCH 1/1] Makefile: Support merged defconfigs

[Sam Bobroff]

Within the Linux kernel, and several other packages, it's currently
possible to maintain defconfigs as diffs (fragments) against other
configs but this is not possible with buildroot itself.

This patch adds the capability, although using a slightly different
implementation. Files may be added in the normal config directories
($TOPDIR/configs or $BR2_EXTERNAL/configs) with the format
"xxx_defconfig.merge" that contain, one per line, the files to be
passed to merge_config.sh to create the matching xxx_defconfig file
(the first line should contain the base config followed by the
fragments).  The generated defconfig file is then handled as it would
normally be.

Signed-off-by: Sam Bobroff <sam.bobroff at au1.ibm.com>
---
For a discussion about this from the OP build project, see
https://github.com/open-power/op-build/issues/457

A couple of things about the implementation:

* merge_config.sh will only output to a file called ".config" so I've set it up
  to output directly to $TOPDIR/.config, since that's about to be overwritten
  anyway, but I don't feel this is particularly nice.
* It seems strange to pass --defconfig=.config when running conf but I couldn't
  quickly tell if there was something better to do. Using --oldconfig would
  pick up .config by default but I don't know if it would then process the file
  correctly so I went with what seemed safe.
* I realise that there are now four very similar rules that handle defconfigs.
  I could look at factoring them but it didn't seem worth the effort and
  complexity at this point.

 Makefile | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/Makefile b/Makefile
index 71735b5..eb80152 100644
--- a/Makefile
+++ b/Makefile
@@ -845,6 +845,16 @@ defconfig: $(BUILD_DIR)/buildroot-config/conf outputmakefile
 	@$(COMMON_CONFIG_ENV) BR2_DEFCONFIG=$(BR2_EXTERNAL)/configs/$@ \
 		$< --defconfig=$(BR2_EXTERNAL)/configs/$@ $(CONFIG_CONFIG_IN)
 
+%_defconfig: $(BUILD_DIR)/buildroot-config/conf $(TOPDIR)/configs/%_defconfig.merge outputmakefile
+	cd $(TOPDIR)/configs && xargs -a $@.merge -x -P 1 $(TOPDIR)/support/kconfig/merge_config.sh -m -O $(TOPDIR)
+	$(COMMON_CONFIG_ENV) BR2_DEFCONFIG=$(TOPDIR)/.config \
+		$< --defconfig=$(TOPDIR)/.config $(CONFIG_CONFIG_IN)
+
+%_defconfig: $(BUILD_DIR)/buildroot-config/conf $(BR2_EXTERNAL)/configs/%_defconfig.merge outputmakefile
+	cd $(BR2_EXTERNAL)/configs && xargs -a $@.merge -x -P 1 $(TOPDIR)/support/kconfig/merge_config.sh -m -O $(TOPDIR)
+	$(COMMON_CONFIG_ENV) BR2_DEFCONFIG=$(TOPDIR)/.config \
+		$< --defconfig=$(TOPDIR)/.config $(CONFIG_CONFIG_IN)
+
 savedefconfig: $(BUILD_DIR)/buildroot-config/conf outputmakefile
 	@$(COMMON_CONFIG_ENV) $< \
 		--savedefconfig=$(if $(DEFCONFIG),$(DEFCONFIG),$(CONFIG_DIR)/defconfig) \
-- 
2.1.0