rootfs-postcommands.bbclass: Follow function rename in rootfspostcommands.py
'shadow_sort' was renamed to 'tidy_shadowutils_files' in
rootfspostcommands.py so we reflect this in
SORT_PASSWD_POSTPROCESS_COMMAND. This also creates a deprecation
function for 'shadow_sort'.
Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com> Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>