armv8: semihosting: add a command to load semihosted images

Instead of sprinkling custom code and calls over the Vexpress64
boardfile, create a command that loads images using semihosting
just like we would load from flash memory of over the network,
using a special command:

    smhload <image> <address>

This will make it possible to remove some custom calls and
code and make the boot easier.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>