Add obmc-phosphor-buttons recipe.

Add obmc-phosphor-buttons_git.bb file and enable buttons

TestBy: export TEMPLATCONF=meta-intel/meta-s2600wf/conf/
        bitbake obmc-phosphor-buttons

(From meta-phosphor rev: 0d737656491b85b06ecb0aaa1290276abc98234f)

Change-Id: I09b182b2adc4d245a3b42794a2e133d940074d55
Signed-off-by: Kuiying Wang <kuiying.wang@intel.com>
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>