Clean up V4L2 codec-related flags and defines

Clean up V4L2 codec-related flags and defines

All arm platforms support V4L2 SVDA by default and use_v4l2_codec also
governs V4L2 VDA/VEA now.
Generates libv4l2 stub in GN file.
This CL is re-landed because CL 828063009 is reverted by 902773003.

BUG=449883, 455545
TEST=build chrome and vda unittest on peach_pit, veyron_pinky, squawks,
and panther_freon
TBR=piman@chromium.org,posciak@chromium.org

Committed: https://crrev.com/5159267af557f4bfab7ff750e4be69d12f452d6c
Cr-Commit-Position: refs/heads/master@{#315226}

[henryhsu, 2015-02-06 17:17:24]

The CQ bit was checked by henryhsu@chromium.org

[commit-bot: I haz the power, 2015-02-06 17:18:33]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/902143002/1

[commit-bot: I haz the power, 2015-02-06 17:18:34]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-02-06 17:18:35]

No LGTM from a valid reviewer yet. Only full committers are accepted.
Even if an LGTM may have been provided, it was from a non-committer,
_not_ a full super star committer.
See http://www.chromium.org/getting-involved/become-a-committer
Note that this has nothing to do with OWNERS files.

[Pawel Osciak, 2015-02-08 04:07:23]

The CQ bit was checked by posciak@chromium.org

[Pawel Osciak, 2015-02-08 04:07:23]

lgtm

[commit-bot: I haz the power, 2015-02-08 04:07:48]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/902143002/1

[commit-bot: I haz the power, 2015-02-08 05:01:07]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-08 05:01:47]

Patchset 1 (id:??) landed as
https://crrev.com/5159267af557f4bfab7ff750e4be69d12f452d6c
Cr-Commit-Position: refs/heads/master@{#315226}