Add the GEOM_LABEL option to the BEAGLEBONE, GUMSTIX, and RPI-B
kernel configuration files, resolving an issue where the UFS and
MSDOSFS partitions would not mount as set in fstab(5).

This is a direct commit to stable/10, as the GEOM_LABEL option
is handled differently in head for arm/armv6. The WANDBOARD
and PANDABOARD already have this kernel option entry via the IMX6
kernel configuration file, so do not need to be changed.

Approved by: re (kib)
Sponsored by: The FreeBSD Foundation