Soong configuration variables

Soong configuration variables are normally set through the following code:

SOONG_CONFIG_ns += var
SOONG_CONFIG_ns_var = x

However, this has an issue where it's difficult to tell where the deliniation between the namespace and variable name is, as both the namespace and variable can contain underscores in their names.

Instead, use the soong_config_get/set/append macros:

$(call soong_config_set,ns,var,x)
$(call soong_config_get,ns,var)
$(call soong_config_set,ns,var,y)