meta/lib/oe/rootfs.py: dont' remove any packages by default

[Robert Yang <liezhi.yang@windriver.com>]

Currently, the rootfs.py removes base-passwd, shadow, update-rc.d,
update-alternatives and run-postinsts when package-management not
in IMAGE_FEATURES, this causes two problems:

1) This makes we can't install the removed pkgs to rootfs, such as
    IMAGE_INSTALL_append = " shadow", the shadow can't installed (first
    installed, then removed)

2) The base-passwd has been removed, but the /etc/passwd and /etc/group
    are still existed since they are generated by preinst, this would
    confuse the user, and we can't add a postuninst to remove /etc/passwd
    and /etc/group since they are required when runtime.

I think that we should not remove any pkgs by default, we can add some
interfaces/ways to let the user decide whether to remove them or any
other pkgs, for example, add a REMOVE_PACKAGS variable, leave it as NULL
or only add run-postinsts to it by default.

-- 
Thanks

Robert