classes/multilib: ensure MLPREFIX is set for image recipes
We need MLPREFIX to be set so that oe.utils.prune_suffix() (as used for
the value of BPN) can derive the bare name from the multilib-extended
name for image recipes. BPN being set correctly avoids missing file
warnings during parse from the file checksum code for (unusual) images
that set SRC_URI, such as build-appliance-image.
First half of the fix for [YOCTO #3146].
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
