[ar71xx] migrate all of the duplicate configuration out into a shared config file.

This brings the default configurations (drivers, net80211 settings, etc) and some
of the shared configuration into std.AR_MIPS_BASE. I haven't yet moved the
-current settings (witness, memguard, etc) into it.

This should simplify building a lot of the same test images for my MIPS AP board
development and testing.

This is a work in progress; it's not designed to be perfect!