wic: change expand behaviour to match docs

wic: change expand behaviour to match docs

The documentation says that --expand takes a comma-separated list of
partition:size pairs, but the code was splitting on hyphens.

Hyphens are not a transitional separator for a list of items, so change the code
to reflect the documentation.

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>