Log warnings to stdout instead of stderr

Currently, both stdout and stderr are redirected to a file. We want
stderr to be visible on the terminal in case the release config
fails, but we don't want to see the spam of warnings release config
always produces.

Move the warnings to stdout so they stay in the file when we start
showing stderr.

Test: m nothing
Change-Id: Ic869675f917270a472142b6e3a4210acaad7499b