sysctl: add cold_boot sysctl entry

Add a cold_boot parameter which supplements the
boot_reason sysctl entry with information about
whether the system was booted from cold or warm state.

/proc/sys/kernel/cold_boot entry is updated with 1 or 0 when
system was booted from cold or warm boot state respecitively.

CRs-Fixed: 461256
Change-Id: I2bc5d80c8f26eb9e9dbb4b34960d991a51a224e4
Signed-off-by: David Keitel <dkeitel@codeaurora.org>
[abhimany: fixup minor merge conflict and drop changes to
kernel/sysctl.c and Documentation since it was brought in via
snapshot commit]
Signed-off-by: Abhimanyu Kapur <abhimany@codeaurora.org>