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".

Change-Id: I94a3cea99d0453249d661a709ecebda030f7f93c
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>
Git-commit: 63a91033
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git


Signed-off-by: Mohammed Khajapasha <mkhaja@codeaurora.org>
Signed-off-by: Prasad Sodagudi <psodagud@codeaurora.org>