base-files: add sysupgrade -u to skip unchanged files

base-files: add sysupgrade -u to skip unchanged files

With '-u', for a file /aaa/bbb/ccc enlisted for backup,
it will only get into backup if /rom/aaa/bbb/ccc does not
exist or /aaa/bbb/ccc is different from /rom/aaa/bbb/ccc.

It also works with '-c', but only effective for files touched
but not modified.

Signed-off-by: Luiz Angelo Daros de Luca <[email protected]>