wic: selftest: avoid COMPATIBLE_HOST issues

wic: selftest: avoid COMPATIBLE_HOST issues

Wic tests will unconditionally attempt to build images that may require
dependencies that are incompatible with current target.

Resolve this by consulting HOST_ARCH first (which defaults to TARGET_ARCH)
before proceeding to build images that may be incompatible.

A convenience decorator only_for_arch() can be used to skip test cases for
specific architectures.

Signed-off-by: Maciej Borzecki <maciej.borzecki@rndity.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>