This is required for developing and testing the binary repository
scripts (see #64). Changes:
* When specified, the local folder gets mounted inside the chroots
as /mnt/postmarketos-mirror
* The apkindex_files() function outputs the correct path to the local
repository (it does *not* hash the URL in that case, which would
be wrong)
* /etc/apk/repositories: when the pmOS mirror is a local folder,
the path "/mnt/postmarketos-mirror" gets added to that file instead
of the outside path (so apk finds it properly inside the chroot)
Oliver Smith