kbuild: add generic mergeconfig target, %.config

"scripts/kconfig/merge_config.sh && make oldconfig" works well
enough for merging local config fragments, but Kbuild currently has
the entry points only for "kvmconfig" and "tinyconfig".

This commit provides the generic target for mergeconfig, so we can
manage our own config fragments easily:
put "foo.config" in arch/$(SRCARCH)/configs/ or kernel/configs/,
and then run "make foo.config".

Now "make kvmconfig" is just a shorthand of "make kvm_guest.config".
Likewise, "make tinyconfig" is equivalent to
"make allnoconfig tiny.config".

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Reviewed-by: Darren Hart <dvhart@linux.intel.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>