hawkboard/omapl-138: Add support for generating ais image for hawkboard

Parameters used for configuring certain SoC peripherals are parsed
from the cfg file and appended as part of the ais image's header. The
u-boot-spl.ais generated is flashed separately to the nand, so do not
delete the file after generation of u-boot.ais.

Signed-off-by: Sughosh Ganu <urwithsughosh@gmail.com>