Add TARGET_ARCH var for top-level arch.

ARCH was overloaded. It was being used in packages
and at the top-level.  A package may be a specific
arch while the overall target system arch is something
else.  This new var describes the target host arch.
SPlit live media ks from installable media.

There are now differences in the *modified*
live media kickstart that don't need to be or
shouldn't be present in the installable media
kickstart, e.g. adding the eject package.

This patch generates two separate kickstarts
from the single kickstart, one for each ISO
target.