package/skeleton: ensure custom skeleton uses merged /usr if needed

When using systemd, the policy in Buildroot is to use a merged /usr
(see c5bd8af6, "system: add options for /bin /sbin and /lib to be
symlinks into /usr" for more info). So, we apply a few tricks in some
packages to account for the merged /usr case.

However, when using a custom skeleton, we have no say in how that
skeleton is organised, so it may well have a split /usr. In that case,
our little tricks might not work as expected.

So, when the user uses a custom skeleton and wants systemd as an init
system, we must check that the custom skeleton is setup with a merged
/usr.

We do that by checking that each pair of {/lib,/usr/lib} {/bin,/usr/bin}
and {/sbin,/usr/sbin} have the same inode numbers, i.e. /lib must have
the same inode number as /usr/lib (and so on...). When a pair does not
share the same inode number, this is not a merged /usr and we abort.

We implement that check with make constructs, so it is done very early
in the build process, and we can abort early if need be.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>