package/skeleton-custom: rework the merged_usr and building conditions

skeleton-custom.mk contains a huge condition for BR_BUILDING and for
BR2_ROOTFS_MERGED_USR. This makes the code difficult to follow, so
rework the condition a bit so that only the parts that really need to
be conditional are indeed conditional.

Note that there is no overhead in needlessly evaluation the "stat"
commands. Indeed, the assignments use late evaluation so the "stat"
is only executed when evaluating the condition - when skeleton-custom
is not selected, stat is never called.

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>