Mount tmpfs in chroot's /dev folders

Created by: ollieparanoid

Mount tmpfs inside the chroot's dev folder to make sure we can create device nodes, even if the filesystem of the work folder does not support it. Closes #1175 (closed).

@ethanrjones97 wrote in https://github.com/postmarketOS/pmbootstrap/issues/1175#issuecomment-371958793:

I managed to port a device over with the patch applied, that seems to be a pretty good test :man_shrugging:

Thanks, that is some solid testing indeed!

I'm also putting this up for review as usually. Would you or someone else take a look at the code before it gets merged?