firmware: split firmware fallback functionality into its own file

The firmware fallback code is optional. Split that code out to help
distinguish the fallback functionlity from othere core firmware loader
features. This should make it easier to maintain and review code
changes.

The reason for keeping the configuration onto a table which is built-in
if you enable firmware loading is so that we can later enable the kernel
after subsequent patches to tweak this configuration, even if the
firmware loader is modular.

This introduces no functional changes.

Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>