kbuild: use -fmacro-prefix-map to make __FILE__ a relative path

The __FILE__ macro is used everywhere in U-Boot to locate the file
printing the log message, such as WARN_ON(), etc.  If U-Boot is
built out of tree, this can be a long absolute path.

This is because Kbuild runs in the objtree instead of the srctree,
then __FILE__ is expanded to a file path prefixed with $(srctree)/.

A brand-new option from GCC, -fmacro-prefix-map, solves this problem.
If your compiler supports it, __FILE__ is the relative path from the
srctree regardless of O= option.  This provides more readable log,
more reproducible builds, and smaller image size.

[ Linux commit: a73619a845d5625079cc1b3b820f44c899618388 ]

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Simon Glass <sjg@chromium.org>