classes/multilib: ensure MLPREFIX is set for image recipes

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].

(From OE-Core rev: ddec9a1b45159c75e97e92abe9a940268acd84b2)

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