Update prebuilt logic to use more prebuilts when available.

- Use chrome_internal prebuilts for internal builds when available.
- Use daisy prebuilts for chromeos-exynos5 boards, but not for other
  arm boards.
- Document that we don't use arm-generic prebuilts for other platforms,
  since they're not binary-compatible.

TEST=Verify external prebuilts are still used by default.
TEST=Verify internal prebuilts are used when USE=chrome_internal
TEST=Trybot runs of paladin and incremental builders.
Change-Id: I667c68b767c2127297080ec3c2f23e8a01894b3c
Reviewed-on: https://gerrit.chromium.org/gerrit/50058
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: David James <davidjames@chromium.org>
Tested-by: David James <davidjames@chromium.org>
2 files changed