product.prop: Use the ?= operator so BT properties can be overridden

System properties set with the "=" operator cannot be overriden. This
device is used elsewhere in the system and some of the properties need
to be overridden. The "?=" operator will use the assigned value if no
other values are assigned elsewhere, allowing for overrides.

Copied from Coral.

Signed-off-by: Amit Pundir <amit.pundir@linaro.org>
Change-Id: I94f12152e778531525e7cdc6b0c9f835b9f665e4
1 file changed