coreboot: Add support for include-what-you-use

The tool "include-what-you-use" analyzes each file's headers and makes
recommendations for header files to add and remove.  There are
additional scripts as part of the package that will make these changes
directly based on the recommendations, but due to the way coreboot
compiles code in/out base on Kconfig options, this isn't really safe for
the project to use.

It is a good starting point though.

To use, set the IWYU kconfig option, then build with the command:

make -k

Because this doesn't actually build any files, the -k option is needed
or make will stop after looking at the first file.

Signed-off-by: Martin Roth <martin.roth@amd.corp-partner.google.com>
Change-Id: I084813f21a3c26cac1e4e134bf8a83eb8637ff63
Reviewed-on: https://review.coreboot.org/c/coreboot/+/67915
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-by: Julius Werner <jwerner@chromium.org>

toolchain.inc

@@ -32,6 +32,21 @@ HOSTCC:=CCC_CC="$(HOSTCC)" $(CC)
 HOSTCXX:=CCC_CXX="$(HOSTCXX)" $(CXX)
 endif
 
+# include-what-you-use integration
+ifeq ($(CONFIG_IWYU),y)
+
+IWYU:=$(word 1,$(wildcard $(addsuffix /iwyu,$(subst :, ,$(PATH)))))
+ifeq ($(IWYU),)
+$(error include-what-you-use selected, but not found in PATH)
+endif
+
+$(foreach arch,$(ARCH_SUPPORTED), \
+	$(eval CC_$(arch):=$(IWYU) ))
+
+CFLAGS_common := -Xiwyu --prefix_header_includes=remove  -Xiwyu --no_comments  -Xiwyu --no_fwd_decls $(CFLAGS_common)
+NOCOMPILE := 1
+endif
+
 COREBOOT_STANDARD_STAGES := decompressor bootblock verstage romstage ramstage
 MAP-decompressor := bootblock