buildhistory: fix not recording SDK information
After OE-Core revision
baa4e43a29e45df17eaa3456acc179b08d571db6 we lost
recording SDK the contents in buildhistory. This was due to the
SDK_POSTPROCESS_COMMAND variable being set with = in
populate_sdk_base.bbclass which overwrote any value set with += in
buildhistory.bbclass; to fix it, use _append in buildhistory.bbclass
instead.
Fixes [YOCTO #8839].
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
