Generate board-specific launchers in mk2rbc

Board config works slightly differently from product config.
It requires the product config variables to be passed into
it. Currently the only generic way to pass info into an RBC
script is via the arguments to rbcrun, which get passed
as global variables, not the product (cfg) variables.
Add a new form of launcher that reads the product variables
from a separate rbc file that is generated in make.

The board configuration also doesn't need inheritance, so it
doesn't call rblf.product_configuration() either.

Bug: 201700692
Test: build/bazel/ci/rbc_product_config.sh -b sdk_phone_x86_64-userdebug
Change-Id: I52fd65b33cf99b45a563284e2849da75a8af8688