This commit refactors the use of Path.which in several parts of the
code. Since dd4d2ed78 the Path utility is capable to run Path.which
on certain chroot env, thus no need to adapt the PATH environment
variable.
Fixes#1281
The preparation to call zipl and the call itself were wrong.
For whatever reason the kernel image the initrd are moved
to another location prior to calling zipl. That move broke
the system because no kernel/initrd existed at the expected
place anymore. In addition the zipl call itself was issued
from a the wrong directory. Also no config file was written
as an after effect of the refactoring in Issue #1194. This
Fixes#1173 and bsc#1156694
