bcm53xx: allow sysupgrade firmwares without UBI containing rootfs

In most cases it allows reverting back to the vendor firmware (as they
usually don't use UBI). If users wants to do that we can't do anything
anyway. Erease counters will be just lost. The only thing we do is warn:
"Flashing firmware without UBI for rootfs. All erase counters will be
lost."
It still requires forcing sysupgrade.

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

SVN-Revision: 45392