expose serial device and speed in runtime configuration

target/config/Config.in.runtime

@@ -127,6 +127,7 @@ config ADK_RUNTIME_GETTY_SERIAL
 	  Start getty on serial console.
 
 config ADK_RUNTIME_CONSOLE_SERIAL_DEVICE
+	prompt "serial console device"
 	string
 	default "hvc0" if ADK_TARGET_SYSTEM_QEMU_PPC64
 	default "ttyUL0" if ADK_TARGET_QEMU_MICROBLAZE_MODEL_S3ADSP1800
@@ -137,6 +138,7 @@ config ADK_RUNTIME_CONSOLE_SERIAL_DEVICE
 	default "ttyS0"
 
 config ADK_RUNTIME_CONSOLE_SERIAL_SPEED
+	prompt "serial console speed"
 	string
 	default "9600" if ADK_TARGET_SYSTEM_FON_FON2100
 	default "38400" if ADK_TARGET_SYSTEM_PCENGINES_WRAP