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.

(From OE-Core rev: a210e28bb3fd5433ebecf50e218fc548013b35dc)

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

scripts/wic

@@ -426,7 +426,7 @@ def expandtype(rules):
     if rules == 'auto':
         return {}
     result = {}
-    for rule in rules.split('-'):
+    for rule in rules.split(','):
         try:
             part, size = rule.split(':')
         except ValueError: