kbuild: fixdep: optimize code slightly

If the target string matches "CONFIG_", move the pointer p
forward.  This saves several 7-chars adjustments.

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