Add support for products overriding properties in /default.prop

We need this to allow products to specify the default USB configuration,
and allow the build system to add adb to this configuration
for eng and userdebug builds.

For example:

PRODUCT_DEFAULT_PROPERTY_OVERRIDES += \
	persist.sys.usb.config=mtp

Change-Id: I83d7d918e57a8117eb96919e17d8ad52afdc0388
Signed-off-by: Mike Lockwood <lockwood@android.com>