fs/initramfs: cleanups, enhance comments

fs/initramfs: cleanups, enhance comments

rootfs-initramfs is not using the generic fs infrastructure, because
there is virtually nothing to do to build the initramfs image: there is
no actual image to be built to begin with.

The only purpose of rootfs-initramfs is to ensure the rootfs.cpio image
is built and then that the Linux kernel is rebuilt with that rootfs.cpio
as initramfs source.

Using variables of the fs infra like if it were used is misleading. It
looked nice as long as there was the possibility that rootfs-initramfs
would one day use the fs infra. But there's no way that will happen any
time soon.

Furthermore, the linux' rule linux-rebuild-with-initramfs now already
depends on rootfs-cpio by itself, so we need not duplicate this
dependency in rootfs-initramfs.

Still, we want to advertise that the dependency is on rootfs-cpio, so
we get nice dependency graphs (and not expose the internal
linux-rebuild-with-initramfs rule to the users).

So, remove the variables and directly define the rules.

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