classes/image: improve debug-tweaks ssh server configuration

classes/image: improve debug-tweaks ssh server configuration

Create a single postprocessing function that enables no-password logins
for both openssh and dropbear when debug-tweaks is in IMAGE_FEATURES,
changing its behaviour slightly:
* Run it regardless of whether ssh-server-* are in IMAGE_FEATURES so
  that it still takes effect if these are installed by adding
  dropbear/openssh to IMAGE_INSTALL.
* Enable it to be run from image.bbclass rather than core-image.bbclass
  so that it works for images that are using the former.

Second half of the fix for [YOCTO #2578].

(From OE-Core rev: 0937054e6e3e02565f57e60a8bdc14b0ad62e249)

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