check-package: enable for fs/

check-package: enable for fs/

The filesystem types can benefit from this script to prevent common
mistakes when submitting patches.

In order to accomplish this:
Do not ignore anymore files from the fs/ directory.
Ignore fs/common.mk as it declares a package infra and not a package itself.
Register the ROOTFS_ as a valid prefix for variables.
Ignore these symbols:
 - PACKAGES_PERMISSIONS_TABLE: defined either by packages through
   pkg-generic or by filesystem types, used by fs/common.mk;
 - SUMTOOL: defined by package mtd, used by filesystem jffs2;
 - TARGETS_ROOTFS: defined by filesystem types, used in the main
   Makefile.

Keep using loose checks that warn about common mistakes while keep the
code simple.
As a consequence the check functions do not differentiate between
packages and filesystems so the symbol PACKAGE_UBI would not generate a
warning for the ubi filesystem neither the symbol ROOTFS_MTD would
generate a warning for the mtd package. But those kind of mistakes are
not common and are obvious in the code review, unlike typos i.e.
ROOTFS_UBl or PACKAGE_MID that would be hard to see in the code review.

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>