skeleton-custom: use a script to check merged usr structure

skeleton-custom: use a script to check merged usr structure

Introduce support/scripts/check-merged-usr.sh, a script that check if a
given path complies to the merged /usr requirements:

    /
    /bin -> usr/bin
    /lib -> usr/lib
    /sbin -> usr/sbin
    /usr/bin/
    /usr/lib/
    /usr/sbin/

Use this script in skeleton-custom.mk instead of a bunch of variables
filled by $(shell ...) macros. The same script will be used to check
rootfs overlays, in a forthcoming change.

Signed-off-by: Carlos Santos <casantos@datacom.ind.br>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>