right. A small oversight on my side. This variable is 'set' in meta-rpb/conf/distro/include/rpb.inc, so whatever is put in local.conf will be ignored. I just fixed this issue in master branch, see
Alternatively, you can do in local.conf
LICENSE_FLAGS_WHITELIST_append = " commercial_mplayer2"
It should set it properly.
However after fixing this, you will end up with another issue:
ERROR: Nothing PROVIDES 'mplayer2'
ERROR: mplayer2 was skipped: Recipe is blacklisted: Not compatible with currently used ffmpeg 3
Which is because of this:
That needs to be investigated..