mboot64: add new FS interface

...implement it for semihosting FS and use it for env load/save
for RTSM64 board.

Paths are like iface:[dev[:part]]/path/to/file, e.g.:

mmc:1:0//vmlinux <-- MMC device 1, partition 0, file /vmlinux
mmc:0/vmlinux    <-- MMC device 0, partition 0, file vmlinux
semi:/env        <-- Semiosting, file env

Now onto the generic loader...

Signed-off-by: Andrei Warkentin <andrey.warkentin@gmail.com>
mboot64: Make bootelf use the FS interface.

Now to boot a file you can:
> bootelf semi:/hello.axf

Or the old way (before semiload goes away)
> semiload 82000000 hello.axf
Loaded <len>.
bootelf mem:/len@82000000

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