Change the /etc/mtab symlink to use an absolute path

As systemd checks if /etc/mtab is a symlink to /proc/mount or
/proc/self/mounts, we need to change it so that we can run systemd.

http://cgit.freedesktop.org/systemd/tree/src/main.c#n1082



Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>