image.bbclass: Also uninstall update-alternatives and shadow for read-only-rootfs
If the rootfs is read-only then we aren't going to be updating any
alternatives or modifying the password file and these binaries will
be redundant. In an ideal world we would be able to stop them from
being installed in the first place but this is non-trivial to arrange.
As a workaround in the meantime, let's just uninstall them once image
construction is finished.
Signed-off-by: Phil Blundell <pb@pbcl.net> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
