printk: Make the console flush configurable in hotplug path

The thread which initiates the hot plug can get scheduled
out, while trying to acquire the console lock,
thus increasing the hot plug latency. This option
allows to selectively disable the console flush and
in turn reduce the hot plug latency.

Change-Id: I42507804d321b29b7761146a6c175d959bf79925
Signed-off-by: Mohammed Khajapasha <mkhaja@codeaurora.org>
Signed-off-by: Rishabh Bhatnagar <rishabhb@codeaurora.org>