Use BOARD_PROPERTY_OVERRIDES_SPLIT_ENABLED for system props split

System props split should be controlled regardless of kernel config
because generic AOSP system image should not include any overrides
but it will be built without kernal.

Additionally we don't have any usecase only to split system build
properties, so we don't need to have two different flags for default
and build properties.

Test: building succeeded and tested with sailfish
Bug: 34116668
Change-Id: I24e909c533da509583a7e8be01f329f52d3820fc