Fix the BEAGLEBONE image build on stable/11.
r336726 merges r336598, r336599, r336600, and r336721, however
there was a mismerge in the BEAGLEBONE.conf where two references
to UFSMOUNT remained.
This is a direct commit to stable/11, as svn:mergeinfo had already
been recorded.
Sponsored by: The FreeBSD Foundation