printk: Defer CPU_ONLINE console flushing

The flushing of the console in the CPU_ONLINE callback on a hotplug
is rather slow, taking up to 11 ms to complete. Defer this to lower
hotplug latency.

Change-Id: If96089fe2c6cd6024d1a9e2bac35296aea75cf9e
Signed-off-by: Jeff Ohlstein <johlstei@codeaurora.org>
(cherry picked from commit 6517f202c7d255a5d315d869605b506516e13a05)

Conflicts:

	kernel/printk.c