oe-selftest-image: use explicit IMAGE_FEATURES instead of debug-tweaks

oe-selftest-image: use explicit IMAGE_FEATURES instead of debug-tweaks

debug-tweaks is vague and doesn't give any indication that the root user
can login over SSH without a password.  This behaviour is incredibly
dangerous if used unwittingly, so discourage it by using the underlying
features explicitly to spell out exactly what is being done.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>