There are many source files shared between U-boot image and tools.
Instead of adding a lot of dummy wrapper files that just include
the corresponding file in lib/ or common/ directory,
Makefile should automatically generate them.

The original inspiration for this came from
scripts/Makefile.asm-generic of Linux Kernel.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>