Moved installation of manjaroiso and symlinking of f123iso into separate prepare script

Current workflow on a newly installed system should look like
git clone
cd
./prepare.sh
And then either build a single iso using 'sudo buildiso' from the top level of the desired profile or
./build.sh
at the top level of the git tree where prepare.sh is located.

Build.sh now runs prepare.sh before attempting to build any isos.
This behavior may change in future; it should not be relied upon yet.