Previously, this function replaces the root password with '*' if
'debug-tweaks' is not in IMAGE_FEATURES. It not only zaps empty root
password, but also zaps non-empty root password. That means, if the
user uses a bbappend file for base-passwd to set the root password, he
would not be able to login as root; if the user uses 'EXTRA_USERS_PARAMS'
to set the root password, he would still not be able to login as root.
What we really want from this function is to disallow empty root password
if 'debug-tweaks' is not in IMAGE_FEATURES. This function should not remove
non-empty root password because that password is usually deliberately set
by the user.
This patch renames zap_root_password to zap_empty_root_password to
better reflect the intent of this function. It also modifies the code
to make this function work correctly.
