Download firmware without using git

This removes the requirement for git. We never make use of partial git updates,
and it is quicker to download the tarball with curl than use git.

Also piping the curl output into tar is quicker than downloading then un-tarring.

Remove the verbose options from the copies (they don't fail, slow down the copies
and swamp any more useful output messages).

By default delete /root/.rpi_firmware (as it's easy to recreate). Use
SKIP_REPODELETE=1 if you want to keep it.

Add option SKIP_BACKUP=1 if you don't want a backup of old firmware and modules.