The method _uninstall_unneeded uses the file installed_pkgs.txt,
this file is left after the build and can cause confusion. This
changes allow to get the installed packages using functions of
rootfs instead of the installed_pkgs.txt file. With this change
now is possible to remove the file without breaking anything.
[YOCTO #8444]
Signed-off-by: Mariano Lopez <mariano.lopez@linux.intel.com> Signed-off-by: Ross Burton <ross.burton@intel.com>
