printk: Add LOG_BUF_MAGIC

Enable post-mortem ramdump analysis to retrieve the valid portions of the
log buffer in the event that portions are corrupted.

Change-Id: Icc47bdb5c030d8548509d14c8016892cc393dafa
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>
[abhimany: resolve trivial merge conflicts]
Signed-off-by: Abhimanyu Kapur <abhimany@codeaurora.org>