verification/linux: run Frama-C on Linux kernel sources

This is mainly a work-in-progress project to verify Linux kernel with
Frama-C, and even without introducting any ACSL code there are already
some bugs... (cf. the patches marked "upstreamable").

This commit makes it possible to run Frama-C value plugin on a kernel
compiled with ARCH=um (UserMode Linux).  Some patches were sent to
upstream (https://lkml.org/lkml/2014/10/12/38) with no reply :(
mbr: move to a new boot/ directory
Makefile: rework recursive subdirectory backlisting
boot/efi: add basic infrastructure to build HelloWorld program
boot/efi: add an UEFI variables dumper