fs: use a per-rootfs fakeroot script

fs: use a per-rootfs fakeroot script

... and locate that script in a per-rootfs directory.

Just like for ROOTFS, this variable will leak down the dependency tree to
target-finalize and packages - But it doesn't matter as it isn't used
outside fs/.

[Peter: extend commit message]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>