Export the board variable to userland using sysenv. This
allows rc-scrips to perform device specific initialisation.
The board variable follows the following pattern
[ARCH]-[ARCHVARIANT]-[VENDOR]-[BOARD]-[BOARDVARIANT]
We currently we support the following boards:
X86-I586-GENERIC-GENERIC-GENERIC
ARM-ARMV7-TI-BBXM-GENERIC
ARM-ARMV7-TI-BB-WHITE
ARM-ARMV7-TI-BB-BLACK
Change-Id: I9e5f5f24f9a71cc9797cacb1aafb19499613f0be