init-install-efi.sh: Avoid /mnt/mtab creation if already present

The base-files recipe installs /mnt/mtab (it is a softlink of /proc/mounts),
so if an image includes the latter, there is no new to created it again inside
the install-efi.sh script, otherwise an error may occur as indicated on the
bug's site.

[YOCTO #7971]

Signed-off-by: Leonardo Sandoval <leonardo.sandoval.gonzalez@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Armin Kuster <akuster808@gmail.com>