fw_loader: Split from fs_loader into separate library file

fw_loader: Split from fs_loader into separate library file

The script based firmware loader does not use anything from the
fs_loader implementation. Separate it into its own library source
file and convert the mediatek PHY to use this separate code. This
should reduce the amount of code that is pulled in alongside the
firmware loader, as the FS loader is no longer included.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>