Device nodes in the chroots get created in a tmpfs, so they can be
created even if the filesystem where the chroot resides does not
support device nodes (#1317). In "pmbootstrap shutdown" we umount the
`dev` folder, which means all device nodes that were created inside
this folder are gone. This commit changes the code to actually recreate
the device nodes when using the chroot again.
Details:
* move `pmb.chroot.init.create_device_nodes` to
`pmb.chroot.mount.crete_device_nodes`
* don't call it in `pmb.chroot.init()` anymore, but in
`pmb.chroot.mount_dev_tmpfs()`
* Create the `null` device as well (`apk --initdb` also creates it on
`init`, but we don't call it after `shutdown`)
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)
