misc/printk: Add a function to get the current hook function.

This can be necessary at run-time if a switch of one console to
another has to be made, with the possibility to reinstall the former
hook at some point.

Change-Id: I60efc74b1c94953bdc2585b99cc4816c46715372
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>