fs: set per-rootfs variable name

fs: set per-rootfs variable name

Like we do for packages with the PKG variable, set ROOTFS to contain the
upper-case name of the rootfs currently being generated.

This will be useful in later patches, when we need more per-rootfs
variables, like a per-rootfs TARGET_DIR for example.

In Makefiles, per-rule variables trickle down the dependency chain, to
all dependencies of that rule, so we have to stop ROOTFS as soon as
we're not in a rootfs. This means we have to stop it at target-finalize
(which is a dependency of all filesystems), and for each package
individually, since some packages (host or target) can be direct
dependencies of filesystems as well.

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>