build_image: use --usepkg instead of --usepkgonly

build_image: use --usepkg instead of --usepkgonly

We want build_image to honour the configuration in PORTDIR and only use
the latest stable version as known by portage and the overlays.

We get these semantics with --usepkg. We do not get them with --usepkgonly.
The one downside is that instead of failing when you forget to run
build_packages before build_image, build_image will try to build
packages from source.

From "man emerge":

       --usepkg[=n] (-k)
              Tells emerge to use binary packages (from $PKGDIR) if they
              are available, thus possibly avoiding some  time-consuming
              compiles.   This option is useful for CD installs; you can
              export PKGDIR=/mnt/cdrom/packages and then use this option
              to have emerge "pull" binary packages from the CD in order
              to satisfy dependencies.

       --usepkgonly[=n] (-K)
              Tells emerge to only use binary packages  (from  $PKGDIR).
              All  the  binary packages must be available at the time of
              dependency  calculation  or  emerge  will  simply   abort.
              Portage  does not use $PORTDIR when calculating dependency
              information so all masking information is ignored.

Change-Id: I267dad8992ac683d0ff4db0d0c72baba61ecbccf

[piman, 2010-06-24 21:58:22]

LGTM

[Christopher Covington, 2010-06-24 22:02:42]

Would a non stable or "workon" package ever get into $PKGDIR?