initramfs-init: adding support for multiple repos when using alpine_repo=

Adding support for comma separated list of URL and directories in the alpine_repo= cmdline option. Adding this support allows packages to be installed from multiple repositories on boot such as main, community, and vendor specific repositories. In addition to directories and URL, this patch allows auto detected repositories to be combined with manually specified repositories.

Example usage is:

alpine_repo=auto,http://foo.org/alpine/edge/main,http://foo.org/alpine/edge/community