oe-core/init-install-testfs.sh: do not overwrite /etc/mtab if the link already exist
Overwriting of /etc/mtab would fail as below if the /etc/mtab link already
exist during installation phase, this patch fix this problem by checking
existance of the link before try to overwrite it.
Error message during installation if the /etc/mtab exists:
"cat: /proc/mounts: input file is output file"
Signed-off-by: Cristian Iorga <cristian.iorga@intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
