App: Change printf to tfm_log_printf

Implement stdio output of NS USART and change the 'printf' in
non-secure App to 'tfm_log_printf'. Use 'tfm_log_printf' from
'interface' folder for the substitute.

Change-Id: I871ea999adb525879cc2f1e3183036feb6d1a671
Signed-off-by: Mingyang Sun <mingyang.sun@arm.com>
4 files changed