scripts/mkknlimg: Improve ARCH_BCM2835 detection

The board support code contains sufficient strings to be able to
distinguish 2708 vs. 2835 builds, so remove the check for
bcm2835-pm-wdt which could exist in either.

Also, since the canned configuration is no longer built in (it's
a module), remove the config string checking.

See: https://github.com/raspberrypi/linux/issues/1157