lib/packagedata: Fix for new overrides

Fix read_subpkgdata_dict to account for the new override operator being
":" instead of "_"

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>