Fix art-host build rule.
Fix art-host build rule.

Use HOST_CORE_IMG_OUT, now that the image is being generated in a
platform dependent directory.

Change-Id: I5509012e9ed0d745b70b2a58227213b027227c97

treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst