check if there is actually anything to extract from the firmware tarball

check if there is actually anything to extract from the firmware tarball
on apple silicon so that we don't fill up the ramdisk by extracting the
whole tarball

while here, change the code so that the machdep.compatible sysctl gets
read once and then that value is used in the script instead of calling
sysctl several times

from input and ok kn@, kettenis@