Makefile: Use relative paths for debugging symbols.

The KBUILD_CFLAGS and KBUILD_AFLAGS variables are adjusted to use
-ffile-prefix-map and --debug-prefix-map, respectively, to use
relative paths for occurrences of __FILE__ and debug paths.

This enables reproducible builds regardless of the absolute path to
the build directory:

  https://reproducible-builds.org/docs/build-path/



Series-to: u-boot

Signed-off-by: Vagrant Cascadian <vagrant@reproducible-builds.org>

Makefile

@@ -607,14 +607,18 @@ endif
 KBUILD_CFLAGS += $(call cc-option,-fno-stack-protector)
 KBUILD_CFLAGS += $(call cc-option,-fno-delete-null-pointer-checks)
 
-# change __FILE__ to the relative path from the srctree
-KBUILD_CFLAGS	+= $(call cc-option,-fmacro-prefix-map=$(srctree)/=)
+# change __FILE__ and debugging symbols to the relative path from the
+# srctree
+KBUILD_CFLAGS	+= $(call cc-option,-ffile-prefix-map=$(srctree)/=)
 
 KBUILD_CFLAGS	+= -g
 # $(KBUILD_AFLAGS) sets -g, which causes gcc to pass a suitable -g<format>
 # option to the assembler.
 KBUILD_AFLAGS	+= -g
 
+# Use relative paths in debugging symbols
+KBUILD_AFLAGS   += --debug-prefix-map=$(srctree)/=
+
 # Report stack usage if supported
 # ARC tools based on GCC 7.1 has an issue with stack usage
 # with naked functions, see commit message for more details