build_mixed script has '-s' option for product.img

For independent system update, we update system.img only.
But the devies that has product partition has some system modules
installed in product partition. In that case, we must update product
partition as well for independent system update.

Use '-s' option without parameters to flash product.img in addition
to system.img for mixed build targets.

Bug: 119266183
Test: development/vndk/tools/build_mixed \
        -p out/prebuilt_cached/artifacts/system/vbmeta.img \
	-s \
	out/prebuilt_cached/artifacts/system \
	out/prebuilt_cached/artifacts/blueline \
	out/dist \
	out/prebuilt_cached/artifacts/system/checkvintf

Change-Id: I86da00630bde7e9e522160c98691087bca9d7d66