zephyr: use SYS_LOG instead of printk

Easier to manage and can be easily disabled via config.

Signed-off-by: Ricardo Salveti <ricardo.salveti@linaro.org>
diff --git a/boot/zephyr/prj.conf b/boot/zephyr/prj.conf
index 2ae9fb9..f3cffde 100644
--- a/boot/zephyr/prj.conf
+++ b/boot/zephyr/prj.conf
@@ -1,5 +1,5 @@
 CONFIG_CONSOLE_HANDLER=y
-CONFIG_PRINTK=y
+CONFIG_SYS_LOG=y
 CONFIG_DEBUG=y
 
 CONFIG_MAIN_STACK_SIZE=10240