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/parse/arguments.py

@@ -260,6 +260,7 @@ def arguments():
     parser = argparse.ArgumentParser(prog="pmbootstrap")
     arch_native = pmb.parse.arch.alpine_native()
     arch_choices = set(pmb.config.build_device_architectures + [arch_native])
+    mirrors_pmos_default = pmb.config.defaults["mirrors_postmarketos"]
 
     # Other
     parser.add_argument("-V", "--version", action="version",
@@ -269,11 +270,11 @@ def arguments():
     parser.add_argument("-c", "--config", dest="config",
                         default=pmb.config.defaults["config"])
     parser.add_argument("-d", "--port-distccd", dest="port_distccd")
-    parser.add_argument("-mp", "--mirror-pmOS", dest="mirror_postmarketos",
+    parser.add_argument("-mp", "--mirror-pmOS", dest="mirrors_postmarketos",
                         help="postmarketOS mirror, disable with: -mp='',"
-                             " default: " +
-                             pmb.config.defaults["mirror_postmarketos"],
-                        metavar="URL")
+                             " specify multiple with: -mp='one' -mp='two',"
+                             " default: " + ", ".join(mirrors_pmos_default),
+                        metavar="URL", action="append", default=[])
     parser.add_argument("-m", "--mirror-alpine", dest="mirror_alpine",
                         help="Alpine Linux mirror, default: " +
                              pmb.config.defaults["mirror_alpine"],