release: Merge the RPI2 and BEAGLEBONE image with the GENERICSD one

Both RPI2 and BEAGLEBONE are still popular and used arm boards.
Both u-boots can coexist as they are named differently and live in the
fat partition.
This leave us with only one image that can be used for both of those
boards and all the other ones supported by FreeBSD provided that you
install the correct u-boot on it.

Reviewed by: imp
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D27283