releasetools: allow for multiple OEM property values.

This CL changes the --oem_settings flag to allow a comma seperated list of
property files. All property values will be used when asserting properties such
as ro.product.name.

For example, if two property files are provided with ro.product.name values of
"sprout" and "sprout_a", the resulting otapackage will check that the device's
ro.product.name property matches at least one of them.

Bug: 34191373
Test: manual

Change-Id: I954673511be8f0929982235cc9cbfbd85a9ee1f4