mboot64: Allow passing commands to evaluate through semihost cmdline.

Generally speeds up using the firmware for other development.

As in the text interface, commands can be separate using ';'.

Ex:

Foundation_v8 --image mboot-rtsm64 --semihost-cmd="env=semi:/my.env eval=\"saveenv;lmb;bootelf semi:/hello.axf semi:/README\"

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