ui/hob: switch from buildFile to buildTargets for custom image builds

ui/hob: switch from buildFile to buildTargets for custom image builds

We need to use the buildTargets command to ensure dependencies, such as
native tools to build the rootfs, are correctly included. This patch
achieves this by modifying BBPATH and BBFILES to include matches for the
location of the generated recipe file and reparsing the metadata before
calling buildTargets.

Fixes [YOCTO #1228]

(Bitbake rev: 5840d59098141e773c12bea8ed8d9f4f1a706132)

Signed-off-by: Joshua Lock <josh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>