stm32l1: desig: use new mechanism to support different densities

Fixes: https://github.com/libopencm3/libopencm3/issues/234

uses the new mechanisms introduced to address a similar problem on F7.

Tested on a medium density part (0x429) that returns the same ids as
before, tested on a high density part that now _doesnt_, but that's now
correct :)

lib/stm32/l1/Makefile

@@ -37,7 +37,7 @@ OBJS += adc.o adc_common_v1.o adc_common_v1_multi.o
 OBJS += flash.o
 OBJS += crc_common_all.o
 OBJS += dac_common_all.o
-OBJS += desig_common_all.o desig_common_v1.o
+OBJS += desig_common_all.o desig.o
 OBJS += dma_common_l1f013.o
 OBJS += exti_common_all.o
 OBJS += flash_common_all.o flash_common_l01.o