printk: introduce printk oops log buffer

The initial kernel oops message often contains informative log messages
to help root cause issues.  However, because of a limited log buffer size,
it is possible that important information is lost due to log buffer
wrap-around.

Introduce oops log buffer which preserves kernel messages from the point
when kernel oops happened by saving messages from regular log buffer when
the initial oops messages wrap arounds.
This guarantees to obtain initial oops and onward messages up to configured
buffer size by running dmesg, /dev/kmsg, /proc/kmsg, kmsg_dumper and other
debugging tools.

Below is a dmesg output example with CONFIG_OOPS_LOG_BUFFER=y after kernel
oops that shows the kernel oops and onward messages up to oops log buffer
while regular log buffer still shows the newest messages with
distinguishing line '---end of oops log buffer---'.

Internal error: Oops: a07 [#1] PREEMPT SMP ARM
Modules linked in:
CPU: 0 PID: 192 Comm: sh Tainted: G        W    3.10.0-gd727407-00072-g7c16272-dirty #87
task: c457ca80 ti: c45dc000 task.ti: c45dc000
PC is at sysrq_handle_crash+0xc/0x14
LR is at write_sysrq_trigger+0x40/0x50
pc : [<c05420ec>]    lr : [<c0542a00>]    psr: 40000013
sp : c45ddf30  ip : fffff100  fp : 0015ab14
r10: 00000000  r9 : 00000002  r8 : 0015bc68
r7 : 00000000  r6 : 00000001  r5 : 0015bc68  r4 : 00000002
r3 : 00000000  r2 : 00000001  r1 : 60000013  r0 : 00000063
Flags: nZcv  IRQs on  FIQs on  Mode SVC_32  ISA ARM  Segment user
Control: 30c5787d  Table: 045673c0  DAC: fffffffd
<snip>
LR: 0xc0542980:
2980  1a000004 e1a00006 e2841034 ebfd8a4d e2707001 33a07000 e1a00007 e28dd004
29a0  e8bd8ff0 c0c41220 c14bfda8 c162a004 c162a008 c146a450 c162a034 c14640c0
29c0  e92d4070 e2524000 0a00000c e1a05001 ebf7b96c e1a0200d e1a00005 e3c23d7f
---end of oops log buffer---
lowmem_reserve[]: 0 9968 9968
HighMem free:1256224kB min:512kB low:2044kB high:3580kB active_anon:132kB inactive_anon:0kB active_file:508kB io
lowmem_reserve[]: 0 0 0
Normal: 9*4kB (UM) 10*8kB (UEM) 6*16kB (UEM) 6*32kB (UEM) 6*64kB (UM) 9*128kB (UEM) 7*256kB (UM) 8*512kB (UM) 7B
HighMem: 0*4kB 2*8kB (UM) 1*16kB (U) 2*32kB (UC) 1*64kB (C) 1*128kB (C) 2*256kB (MC) 2*512kB (MC) 3*1024kB (UMCB
364 total pagecache pages
<snip>

CRs-fixed: 561659
Change-Id: I587961931bfea3aba64a40790598020a4d5b9b36
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>