classes: add stx-image-list.bbclass to generate pkglist

classes: add stx-image-list.bbclass to generate pkglist

Add stx-image-list.bbclass to generate pkglist automatically
so we don't need to maintain the pkglist manually and the
pkg lists in conf/distro/files/pkglist/ are removed.

After this change, the following pkglist file will be generated:
- For target build:
  tmp/deploy/images/intel-corei7-64/stx-image-aio-intel-corei7-64.pkglist
- For anaconda installer build:
  tmp-glibc/deploy/images/intel-corei7-64/stx-image-aio-installer-intel-corei7-64.stx-image-aio.pkglist

Issue-ID: INF-141
Signed-off-by: Jackie Huang <jackie.huang@windriver.com>
Change-Id: I0b183b9e4a2e665c4af6c4f33c3caf8a4f80a93e