testimage.bbclass: check that root-login-with-empty-password image features are present

testimage.bbclass: check that root-login-with-empty-password image features are present

More or less all of testimage relies on logging in as root, without password,
both on console and over ssh. Previously this was enabled by default in poky
and core, but now that it isn't, testimage will error out on timeouts in
both console and ssh login attempts. This commit adds an earlier check and
provides a hint to the users about what they should do.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>