Automatically mount binfmt_misc if necessary

Created by: ollieparanoid

When running pmbootstrap on Alpine, you need to do:

sudo modprobe binfmt_misc
sudo mount -t binfmt_misc none /proc/sys/fs/binfmt_misc

... or you will get:

sh: can't create /proc/sys/fs/binfmt_misc/register: nonexistent directory

We could check if /proc/sys/fs/binfmt_misc/register exists, and if it does not, perform the two commands above.

See also https://wiki.postmarketos.org/wiki/Troubleshooting#sh:_can.27t_create_.2Fproc.2Fsys.2Ffs.2Fbinfmt_misc.2Fregister:_nonexistent_directory