armv8: semihosting: delete external interface

Now that loading files using semihosting can be done using
a command in standard scripts, and we have rewritten the boardfile
and added it to the Vexpress64, let's delete the external
interface to the semihosting file retrieveal and rely solely
on these commands, and staticize them inside that file so the
whole business is self-contained.

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