build: Include LineageOS specific properties in build.prop

 * Our properties were supposed to go to /system/etc/prop.default
    after the following commit:
    "lineage: Move to Google's method of defining system default props"
    Change-Id: I6cb0e28a7599b010b389cc541015a37010a00f4b

 * However if BOARD_PROPERTY_OVERRIDES_SPLIT_ENABLED is not true,
    only /default.prop will retain the properties contents of
    ADDITIONAL_DEFAULT_PROPERTIES and PRODUCT_SYSTEM_DEFAULT_PROPERTIES,
    and none of our versioning identification was held in the system

 * Enabling BOARD_PROPERTY_OVERRIDES_SPLIT_ENABLED globally would
    break all properties on devices that handle partitions usually
    at the device level rather than the kernel due to mounting races

 * Include the vendor/lineage/config/main.mk entrypoint to be allowed
    to extend ADDITION_BUILD_PROPERTIES as we need to define our releases

Change-Id: I19918eece0f0dd3ee967db0339b4d09c9a6d540e
Signed-off-by: Adrian DC <radian.dc@gmail.com>