image.bbclass: Make zap_root_password to disable root's password not the user root...

image.bbclass: Make zap_root_password to disable root's password not the user root itself.

* With shadow now running pwconv after commit 7c5f81b2139e55622ca2f23ff6b63438d4825d87
  It converts :*: passwd entry into equivalent /etc/shadow entry :*:
  which in shadow means disable the account as per
  http://tldp.org/LDP/lame/LAME/linux-admin-made-easy/shadow-file-formats.html
  As a result root can not login unless one boots into shell and then
  resets the password.

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Acked-by: Chris Larson <clarson@kergoth.com>