bcm4908: sysupgrade: add pkgtb format support

bcm4908: sysupgrade: add pkgtb format support

BCM4908 devices with U-Boot use pkgtb firmware format. It's based on
U-Boot's FIT: DTB with configurations, images & embedded data.

This format contains bootfs, rootfs and optionally a first stage U-Boot
loader. Contained images need to be extracted & flashed.

Broadcom used two sets of firmwares: main & backup. It uses UBI volumes
"metadata1" & "metadata2" for storing U-Boot env variables with info
about flashed images.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>