Dump soong config variables in dump-variables-rbc

There are some products whose board configurations
use soong_config_get to read the values of soong config
variables that were set in the product configuration.
These variables were being lost, as dump-variables-rbc
was skipping the soong config variables because mk2rbc
couldn't handle converting the raw SOONG_CONFIG_* variables.
To fix that issue, dump-variables-rbc now dumps them as
calls to soong_config_set instead.

Bug: 201700692
Test: m RBC_BOARD_CONFIG=1 nothing on certain products
Change-Id: I91ca8418635a94cf80362cad1729f48854f6bc98