linux: meddle not in the internals of filesystems

linux: meddle not in the internals of filesystems

Currently, the rule to rebuild the Linux kernel with an initramfs
directly depends on the path of the file of the intermediate cpio image.

This is inherently "bad" from a purity point of view; linux.mk should
not have to delve into the fs internals.

Rather, make it directly depend on the "frontal" rule that generates the
cpio image.

Drop the comment for linux-rebuild-with-initramfs, it was misleading
(talking about generating "the initramfs list of files", which is not
what was done, since we use a cpio as source of initramfs, not a list of
files).

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>