system/skeleton: update etc/mtab with a more sensible link

Currently, our /etc/mtab points to /proc/mounts. This was all neat so
far, and was good for a sysv-like init system.

However, the way today is to point it at /proc/self/mounts, the
per-process mount tab.

Additionnally, that's what systemd expects. If /etc/mtab is not a
symlink to ../proc/self/mounts and the rootfs is readonly, systemd would
whine loudly (and a service unit would be marked failed).

Since it works well for sysv-like init systems too, just use that.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

diff --git a/system/skeleton/etc/mtab b/system/skeleton/etc/mtab
index 4c0a0948bccbaba6ed2204aa7389b361d049bfa7..5c4677a608fb9c8a106185ec7c1a895f60021d17 120000 (symlink)
--- a/system/skeleton/etc/mtab
+++ b/system/skeleton/etc/mtab
@@ -1 +1 @@
-/proc/mounts
\ No newline at end of file
+../proc/self/mounts
\ No newline at end of file