bcm53xx: sysupgrade: extract TRX partitions directly from vendor image

Extracting full TRX out of vendor format is not needed as otrx supports
passing TRX offset. This saves some RAM during sysupgrade.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@45911 3c298f89-4303-0410-b956-a3cf2f4a3e73