base-passwd: Add the sgx group

base-passwd: Add the sgx group

To avoid errors from eudev/udev we need an sgx group, but if we add it
via groupadd that causes shadow login to be brought into an image, which
causes images which have CONFIG_MULTIUSER unset to fail with `setgid:
Function not implemented` as shadow's login doesn't implement the
heuristics which busybox has to handle this kernel configuration.

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>