Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Turns out the nvram env was common, so move it back up to get PB92 working.

Add RT305x specific devices here.

Import the initial CPU support for the MIPS RALink RT305x SoC.
This is a MIPS4KC CPU with various embedded peripherals, including
wireless and ethernet support.

This commit includes the platform, UART, ethernet MAC and GPIO support.
The interrupt-driven GPIO code is disabled for now pending GPIO changes
from the submitter.

Submitted by: Aleksandr Rybalko <ray@dlink.ua>