classes-recipe/core-image: drop debug-tweaks IMAGE_FEATURE

classes-recipe/core-image: drop debug-tweaks IMAGE_FEATURE

Remove the 'debug-tweaks' IMAGE_FEATURE. It sounds friendly and kind to
developers, but it results primarily in an image which root can login
remotely without a password.  This is incredibly useful for local
development and testing purposes, but we really want to be explicit that
this is what is happening instead of hiding it behind a vague "debug
tweaks" statement.

To preserve the eixsting behaviour, debug-tweaks should be replaced with
these features:

  allow-empty-password empty-root-password allow-root-login post-install-logging

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