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