Add functions for easier vendor var definitions.

The add_soong_config_namespace adds a namespace and
initializes it to be empty. The add_soong_config_var
function adds a pre-existing variable as a Soong
config variable, and the add_soong_config_var_value
function defines a make variable and also adds it as
a soong config variable.

Bug: 144532889
Test: $(OUT)/soong/soong.variables contains variables.
Change-Id: I5cc6a670a089ecd5163f19f7a2fba33f5860c952