mboot64: basic secure monitor with some simple ops.

See arch/arm64/include/asm/mon.h

Signed-off-by: Andrei Warkentin <andrey.warkentin@gmail.com>
mboot64: clarify semihost "console"

I don't think the semihosting readc interface is usable for console input.

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