kbuild: consolidate PLATFORM_LIBS

We had switched to Kbuild so now we can specify
PLATFORM_LIBS/PLATFORM_LIBGCC with relative path.

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