Not waiting for loop devices after modprobe loop

Created by: ollieparanoid

When running pmbootstrap install, we create a big empty file and mount it as loopback device, in order to copy the rootfs into it. To do that, we require the loop kernel module, which may not be loaded by default.

In that case we load it with modprobe and expect the loopdevices to directly appear afterwards:

https://github.com/postmarketOS/pmbootstrap/blob/17cdc48329714e86514a04d51ca75b3ec3353c79/pmb/install/losetup.py#L29-L35

But that is not the case, sometimes it will fail at this point:

$ sudo rmmod loop
$ lsmod | grep loop # this should be empty when it's unloaded
$ pmbootstrap install

with:

losetup: /home/pmos/rootfs/qemu-amd64.img: failed to set up loop device: No such file or directory

To fix this, we can add a short loop that waits 100ms until the loop devices are available.

It did not fail on my PC as I've tested it, but it fails on @craftyguy's shared gitlab CI runner every time.

Related: https://github.com/postmarketOS/pmbootstrap/pull/1539#issuecomment-396425973