package/skeleton-custom: also check for missing directories

package/skeleton-custom: also check for missing directories

In addition to checking the symlinks in the merged usr case, also check
whether /bin, /lib, /sbin and their /usr/... counterparts exist in the
non-merged case.

Missing directories are collected in the variable
SKELETON_CUSTOM_MISSING_DIRS. For symmetry and because it's more
logical, rename SKELETON_CUSTOM_NOT_MERGED_USR to
SKELETON_CUSTOM_NOT_MERGED_USR_DIRS.

When a directory is indeed missing, "stat" will print an error.
Buildroot *also* prints an error, which is clearer. So remove the error
from stat by redirecting it to /dev/null.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[Arnout: split off in a separate patch + wrote commit message]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>