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

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

Use DEVMETHOD_END macro defined in sys/bus.h instead of {0, 0} sentinel on device_method_t arrays

Reviewed by: cognet
Approved by: cognet

Import nvram2env, a device driver which imports various NVRAM-style
environments into the kernel environment.

The eventual aim is to replace these with specific drivers for
the various bootloaders (redboot, uboot, etc.) This however will
work for the time being until it can be properly addressed.

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