oe-core/init-install.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: Shan Hai <shan.hai@windriver.com> Signed-off-by: Saul Wold <sgw@linux.intel.com>
