build: Combine "savedefconfig" and "stripped config" in CBFS `config`

The intention of CB:69710 was that the expanded config file introduced
there would be a strict superset of the old version and could be used in
all the same cases. This is generally true except for a small oversight:
if a boolean Kconfig is `default y`, but was manually set to `n` by the
user, the new `config` file does not include a line for it. Running
`make olddefconfig` on such a file will again introduce the option as
`y`. It turns out that `make olddefconfig` actually parses those
"load-bearing comments" in that case.

This patch fixes the problem by also generating the minimal defconfig
(like before CB:69710), and then just appending the non-comment lines
from the full config that don't appear in it already. This ensures that
any "load-bearing comments" in the defconfig remain in the file and the
result of Kconfig utilities regenerating a full config from there will
again be the same as before CB:69710. In addition, it clearly separates
the "minimal defconfig" part of the file from the rest, making it easy
for people to extract that if they need it; while also keeping all the
config values in one file to make it easy to grep for a certain value.

Also eliminate that random backslash in the recipe that doesn't seem to
have any good reason to exist and was probably a typo to begin with.

Signed-off-by: Julius Werner <jwerner@chromium.org>
Change-Id: I52ba5d20d3536498fae79d529acf7135f97ef1a8
Reviewed-on: https://review.coreboot.org/c/coreboot/+/69955
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Jakub Czapiga <jacz@semihalf.com>
Reviewed-by: Yu-Ping Wu <yupingso@google.com>
diff --git a/Makefile.inc b/Makefile.inc
index 3e21bc1..5cd13ba 100644
--- a/Makefile.inc
+++ b/Makefile.inc
@@ -306,7 +306,7 @@
 		mv $(2).tmp $(2))
 
 #######################################################################
-# Reduce a .config file by removing lines about unset booleans
+# Reduce a .config file by removing lines about default unset booleans
 # arg1: input
 # arg2: output
 define cbfs-files-processor-config
@@ -314,8 +314,12 @@
 		+printf "    CREATE     $(2) (from $(1))\n"; \
 		printf "# This image was built using coreboot " > $(2).tmp && \
 		grep "\<COREBOOT_VERSION\>" $(obj)/build.h |cut -d\" -f2 >> $(2).tmp && \
-		sed -e '/^CONFIG/!d' $(1) >> $(2).tmp && \
-		\mv -f $(2).tmp $(2))
+		$(MAKE) DOTCONFIG=$(1) DEFCONFIG=$(2).tmp2 savedefconfig && \
+		cat $(2).tmp2 >> $(2).tmp && \
+		printf "# End of defconfig. Derivable values start here.\n" >> $(2).tmp && \
+		grep "^CONFIG" $(1) | grep -F -v -f $(2).tmp2 >> $(2).tmp && \
+		rm -f $(2).tmp2 && \
+		mv -f $(2).tmp $(2))
 endef
 
 #######################################################################