Separate output from diagnostics in Starlark product configuration.

Some of the product configuration makefiles use `info` and `warning` Make's
builtins for diagnostics. As running Starlark configuration generates the makefile
as its output, this diagnostics has to go elsewhere. Implement `rblf_log` as
the functional equivalent of `print` that writes to stderr instead of stdout
and use it to implement `info` and `warning` counterparts.

Fixes: 201073196
Test: manual
Change-Id: Ib4d9c10566f9b20310fbee41eda67f0a621b0a84