[Makefile] remove mkinitcpio dependency (#136)

Errata commit:
This commit has no substantive code change.
This commit is provided only to document a correction to a previous commit message.
This pertains to commit object 87b8163

Due to improper handling, the aforementioned commit message is incorrect.
Correct message is :

[Makefile] remove `mkinitcpio` dependency

The mkinitcpio binary is not available even on Arch,  when building in a
clean environment (e.g. building distro packages in a clean chroot).

This causes the mkinitcpio hook  to not be present in the final package,
which is resolved by this commit, by adding a `make` switch. This switch
allows users to explicitely turn on inclusion of the hook.

[uninstall:] don't remove non-existent folders

This prevents some warnings being output when on a non-Archlinux system,
without a previously installed initcpio hook, a `make uninstall` is per-
formed.

These three folders should _only_ be deleted if they had been mistakenly
created during `make install`.

Co-authored-by: Kriss <[email protected]>