Split base.mk into system and vendor parts.

Put all packages that end up on the vendor partition in a separate mk,
to allow creating products with artifact path requirements.

Bug: 80410283
Test: diff product vars with multiproduct_kati
Change-Id: Ia35adaed2c8f9989c180bfdbbeeb2a5ecf634aa5
Merged-In: Ia35adaed2c8f9989c180bfdbbeeb2a5ecf634aa5