frontend.config: add support for clearing a config value
Previously these two commands would both print the current value:
pmbootstrap config extra_packages
pmbootstrap config extra_packages ''
With this change, the second command will instead set the given config value to the empty string.
Before:
$ pmbootstrap config extra_packages foo
[10:16:27] Config changed: extra_packages='foo'
$ pmbootstrap config extra_packages ''
foo
After:
$ pmbootstrap config extra_packages foo
[10:29:06] Config changed: extra_packages='foo'
$ pmbootstrap config extra_packages ''
[10:29:07] Config changed: extra_packages=''