Clarify the licenses mentioned in the header files of
depthcharge. While some files are dual-licensed BSD / GPLv2,
the overall product of depthcharge is GPLv2.

Signed-off-by: Stefan Reinauer <reinauer@google.com>
BRANCH=none
BUG=none
TEST=no functional change

Change-Id: Ica2540461acf001c4cffde50c8b8240fe78fee96
Reviewed-on: https://chromium-review.googlesource.com/837589


Commit-Ready: Stefan Reinauer <reinauer@google.com>
Tested-by: Stefan Reinauer <reinauer@google.com>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>