Better Soong config namespace support makefile macros.

Implement two new macros, `soong_config_set` and `soong_config_append`,
which will eventually replace the existing add_soong_config_xxx. With the
new macros, namespace declaration is implicit, and a value can be appended
to a variable in the namespace.

Bug: 200297238
Test: treehugger

Change-Id: I4ffd21bdb6d93db3b1630703fc5f766649b4152c