Support automounting of alpine iso file

Created by: CasperVector

... so that if 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 search all /dev/sd*[0-9] partitions for the alpine iso file at the specified path.