lib/oe/rootfs: Use list_pkgs() instead of list()

lib/oe/rootfs: Use list_pkgs() instead of list()

This patch changes the use list_pkgs() instead of list()
from class RpmPkgsList. The change is in two functions,
image_list_installed_packages from rootfs.py and
sdk_list_installed_packages from sdk.py.

With this change the functions calling the functions
listed above, must format the output as they required.
The formatting can be done using format_pkg_list() from
oe.utils.

The classes calling the afected functions are changed too
with this patch, to keep the same functionality using the
new data structure.

[YOCTO #7427]

Signed-off-by: Mariano Lopez <mariano.lopez@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>