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>