stm32:l0: enable usart peripheral

Now that the usart-v2 peripheral is extracted cleanly, adding it for l0
is very simple.

lib/stm32/l0/Makefile

@@ -47,6 +47,7 @@ OBJS		+= exti_common_all.o
 OBJS		+= flash.o flash_common_l01.o
 OBJS		+= i2c_common_v2.o
 OBJS		+= rng_common_v1.o
+OBJS		+= usart_common_all.o usart_common_v2.o
 
 OBJS            += usb.o usb_control.o usb_standard.o
 OBJS            += st_usbfs_core.o st_usbfs_v2.o