Add mtkclient as a flasher option

Since we're using mtkclient a lot anyways, it makes sense to add
support for it to pmbootstrap.

This was originally implemented by JustSoup321, but they had issues
submitting the patch, so I've cleaned it up for upstream submission.

Co-Authored-By: JustSoup321 <brandonboese@protonmail.com>
Co-Authored-By: Oliver Smith <ollieparanoid@postmarketos.org>
Signed-off-by: hexaheximal <hexaheximal@proton.me>
Reviewed-by: Oliver Smith <ollieparanoid@postmarketos.org>
Link: https://lists.sr.ht/~postmarketos/pmbootstrap-devel/%3C20230712231807.22590-1-hexaheximal@proton.me%3E