Skip backup in rpi-update.

The do_backup function was changed recently in rpi-update:
https://github.com/Hexxeh/rpi-update/commit/41e23a0924d18786ac2374b04277d1223bdec614

Since the kernel isn't installed before rpi-update run, and they don't check that the module folder exists before trying to copy it, this causes an error. Adding the SKIP_BACKUP=1 option avoids the error.