board: introduce base config for TI BeagleBone family boards

board: introduce base config for TI BeagleBone family boards

This patch adds the file ti_beaglebone-base.cfg as the common base
configuration for all TI BeagleBone derived boards. It also modifies
ti_beaglebone.cfg to source the base board and only add the on-board
JTAG adapter. Lastly, it adds a file ti_beaglebone_black.cfg with
a suitable configuraton for the BeagleBone "Black" variant.

Change-Id: I40cacb8abed7bdb308929713891f7b5e5b685c95
Signed-off-by: Matthias Welwarsky <matthias@welwarsky.de>
Reviewed-on: http://openocd.zylin.com/3099
Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>