Crashes that happen before tombstoned is running are extremely hard to
diagnose, because tombstones aren't written to disk, and the window of
opportunity to get logs via `adb logcat` is small (potentially
nonexistent).
Solve this by adding a world-writable /dev/kmsg_debug on userdebug
builds, and writing to it in addition to logcat when tombstoned hasn't
started yet.
Bug: http://b/36574794
Test: stop tombstoned; crasher; dmesg
Change-Id: I46ba2dd67c188be74bd931f8a5536b6342d537f2
3e76ecaf80