Support automounting of alpine iso file

Created by: CasperVector

... so that if isodev=UUID=xxxxxxxx isoloop=/path/to/alpine.iso is provided in the kernel command line (of course, for this to actually work, isofs and perhaps ntfs need to be appended to the modules=... option), the initramfs will, upon failure mounting the boot media, attempt to mount the alpine iso file at the specified path of the specified device.