OpenWrt support for Linksys MX4200

[FanOfOpenWRT]

There's been an ongoing discussion of the TX power limits in OpenWrt, and the consensus is it's not just one easily fixable thing:

@eginnc

If nothing else, the next person to notice they've lost 80% of their U-NII-1 transmit power after flashing OpenWrt may find the actual reason why in this thread - versus hardware limitation, antenna gain, FCC certification, fill-in-the-blank etc. red herrings.