x86: coreboot: delete unused coreboot/config.mk

HOSTCFLAGS_autoconf.mk.dep was added by commit 422322f2
but it has never been used.

Cc: Vadim Bendebury <vbendeb@chromium.org>
Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>