If BUILD_TAG is part of KBUILD_CFLAGS, then any time the value changes,
all files get rebuilt. In a continuous integration environment, the value
will change every build. This wastes time, assuming that incremental
builds would otherwise occur.

To solve this, remove BUILD_TAG from KBUILD_CFLAGS and add it to CFLAGS
for just the one file that uses it. This does have the disadvantage that
if any other files want to use the flag, we'll need to duplicate this
custom CFLAGS setup logic. However, it seems unlikely we'll need this.

An alternative would be to add BUILD_TAG to the "local version" and remove
the special case code from display_options.c. However, that would affect
the format of the U-Boot signon message, which may negatively affect
people looking for specific data there. The approach of using
file-specific CFLAGS was suggested by Masahiro Yamada.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Simon Glass <sjg@chromium.org>