add support for hibernation to swap file

Adds support for hibernating to/resuming from a swap file on alpine linux. In order to resume from a swap file, the kernel has to know the offset of the file from the beginning of the partition. This offset should be specified as a parameter on the kernel command line: resume_offset=<offset>. See kernel documentation.

I changed the initramfs-init script to parse the resume_offset parameter and pass the offset to the kernel.