u-boot: Add to support building PBL image

To enable the feature, insert "UBOOT_PBL_ENABLE = yes" into
$(MACHINEDIR)/machine.make.

Signed-off-by: Curt Brune <curt@cumulusnetworks.com>
Updater can't extract correct 'ver' and 'onie_bootcmd' from u-boot.pbl

The Freescale PBL image has special format that different from
u-boot.bin.  ONIE updater cannot extract 'ver' and 'onie_bootcmd'
from the PBL image easily.

The patch will pack u-boot.bin and u-boot.pbl into the ONIE updater
if 'UBOOT_PBL_ENABLE' is turned on.  The updater will write u-boot.pbl
to MTD device and parse u-boot.bin to get 'ver' and 'onie_bootcmd'.

Signed-off-by: Curt Brune <curt@cumulusnetworks.com>