fix(dracut.sh): --sysroot option broken if global variables not set in conf

fix(dracut.sh): --sysroot option broken if global variables not set in conf

When `$dracutsysrootdir` is set and references an existing directory, the
internal global variables are never set, unless their value is assigned via
configuration file.

The simple fix would be turn all:

```
[[ -d $dracutsysrootdir$var ]] || var=value
```
into:
```
[[ -z $var || ! -d $dracutsysrootdir$var ]] && var=value
```

But this patch also includes a refactoring, removing duplicate code using the
same `set_global_var` function to set and export global variables, and improving
readability.