Move dump-products back to the config stage

It's unnecessary to run all the make file parsing to dump product
configs. Move the dumping to product_config.mk instead, but switch
the output to $(warnings) as stdout is a bit sensitive during
the config stage.

Test: multiproduct_kati -only-config dump-products
Change-Id: I8c7365c9f90ea2bf152b82dfb983bfb0cdcb1697