mboot64: bring back ENV

It would be actually kinda cool to use semihosting to load
the environment in and out.

Signed-off-by: Andrei Warkentin <andrey.warkentin@gmail.com>
mboot64: semihost-backed env for RTSM64

Alright, that wasn't too hard.

Signed-off-by: Andrei Warkentin <andrey.warkentin@gmail.com>