allow specifying multiple postmarketOS mirrors (!1718)

Multiple -mp arguments can be used to list multiple mirrors:
$ pmbootstrap -mp=first -mp=second chroot -- cat /etc/apk/repositories

This is needed for the new build infrastructure, so we can have a WIP
repository to which we push packages until all of them are up to date,
and then publish all of them at once. Software like KDE/Plasma Mobile,
which expect a lot of packages to be updated from one version to
another will not end up with a half-way through upgrade that way.

pmb/helpers/pkgrel_bump.py

@@ -76,9 +76,9 @@ def auto_apkindex_files(args):
         if os.path.exists(local):
             ret[arch].append(local)
 
-        if args.mirror_postmarketos:
+        for mirror in args.mirrors_postmarketos:
             path = (args.work + "/cache_apk_" + arch + "/APKINDEX." +
-                    pmb.helpers.repo.hash(args.mirror_postmarketos) + ".tar.gz")
+                    pmb.helpers.repo.hash(mirror) + ".tar.gz")
             ret[arch].append(path)
     return ret