Fine grained per property /chosen updating.

Implement a suggestion by Scott Wood to make the /chosen handling fine
grained.  Don't overwrite pre-existing properties on a per-property basis,
so if /chosen exists but a necessary /chosen/property doesn't, it gets
created.  If a /chosen property exists, it is NOT overwritten unless the
"force" flag is true.

Signed-off-by: Gerald Van Baren <vanbaren@cideas.com>