prereq: merge common cases in SetupHostCommand

prereq: merge common cases in SetupHostCommand

Now that most cases do the same thing in SetupHostCommand, merge them
together into one. To allow moving the generic symlink check, invert the
check and let it check for relative links by matching on link targets
that do not start with a slash.

This then allows us to also drop the absolute link case, shortening the
case statement further.

This reorders the check to

* if it is not a symlink, do not change it
* if it is a symlink and it points to the found command, do not change it
* if it is a symlink with a relative path, do not change it
* else, update/replace it

Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>