SDK Addon multi-sys-img support, part 4.

SDK Addon multi-sys-img support, part 4.

Change the add-on build rules to packages the system-image
separately from the main add-on zip file. This is then picked
up by development's sdk_repo.mk to generate two repository
packages files (one for the add-on, one for its system image.)

The system-image now also contains a source.propertie file,
which value is not infered from the add-on's manifest.ini

Add-on product files need to be modified to define a
PRODUCT_SDK_ADDON_SYS_IMG_SOURCE_PROP variable that points
to their source.properties or source.prop_template file.

Change-Id: I79e9cdfd43c99f099a70890fb3e5e9215ad647f4