[nrfconnect] Enable logger synchronization (#9457)

nRF Connect examples use the minimal logger mode which
uses printk() under the hood. Enable synchronization
in printk() so that logs witten from multiple threads don't
interfere with each other.

config/nrfconnect/app/sample-defaults.conf

@@ -20,6 +20,7 @@ CONFIG_STD_CPP14=y
 
 CONFIG_LOG=y
 CONFIG_LOG_MODE_MINIMAL=y
+CONFIG_PRINTK_SYNC=y
 CONFIG_ASSERT=y
 CONFIG_HW_STACK_PROTECTION=y
 CONFIG_SHELL=y