axfs: add new filesystem type

The Advanced XIP File System is a Linux kernel filesystem driver that
enables files to be executed directly from flash or ROM memory rather
than being copied into RAM. It has the ability to store individual
*pages* in a file uncompressed/XIP or compressed/Demand Paged.

This commit only adds support for 'XIP all' mode, so all the files that
have the execute attribute set will be XIP'ed.

At the moment, the FS is not supported in Linux mainline (v4.3-rc5), so
the kernel has to be built with the axfs patches to be able to read it.
Patches can be found here: https://github.com/jaredeh/axfs



[Thomas: rewrap Config.in help text.]

Signed-off-by: Ariel D'Alessandro <ariel@vanguardiasur.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>