hack(build_image): Make check for /usr/share/locale a warning.

Switching the toolchain to upstream Gentoo brought this directory back
and based on the Chromium OS history keeping this directory out of the
builds is a bit tedious. Keeping image sizes down isn't *that* important
right now so just let it be.
Merge pull request #42 from marineam/locale

hack(build_image): Make check for /usr/share/locale a warning.