hob/bitbake: custom image is now using the base image

hob/bitbake: custom image is now using the base image

Till now, a custom image made in Hob was using only the packages from
the base image. Now it is using everything declared in the base image.
Also next to hob-image.bb, it creates another .bb file which is used
in building process. Those images are ignored by git.

[YOCTO #2601]
Signed-off-by: Cristiana Voicu <cristiana.voicu@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>