mboot64: Add semihosting interface.

Now onto writing some commands...

Signed-off-by: Andrei Warkentin <andrey.warkentin@gmail.com>
mboot64: Add a load command for semihosting.

RTSM64 # semiload 81000000 README
Loading file "README"
1065 bytes read
RTSM64 # md 81000000 10
81000000: 4f4f424d 20343654 6d726966 65726177    MBOOT64 firmware
81000010: 3d3d3d0a 3d3d3d3d 3d3d3d3d 3d3d3d3d    .===============
81000020: 540a0a3d 20736968 61207369 726f6620    =..This is a for
81000030: 666f206b 65687420 69726f20 616e6967    k of the origina

I did just realize that if you run with semihosting explicitely
off than we crash. Oops. I'll fix this when I make the exception
handlers do something interesting.

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