core-image: allow root login when debug-tweaks is enabled

core-image: allow root login when debug-tweaks is enabled

This allows root to login over ssh with an empty password just like
dropbear when the debug-tweaks are enabled, it's important to disable
debug-tweaks for a production system as this will leave open a security
hole!

Thanks to Marc for the settings.
Cc: Marc Ferland <marc.ferland@gmail.com>
[Yocto #3078]

(From OE-Core rev: 13e6aa8bba6ab1ebba1efa23f94af379a8fcb6a9)

Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>