base/staging: use HOST_PREFIX, not TARGET_PREFIX

This matters when cross-compiling a cross-toolchain:
we need to specify the system where the toolchain
will run, not the system it will produce output for.

For everything else, HOST and TARGET are the same.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>