stm32: flash: pull up prefetch to _all

Turns out, there's lots of common code for flash.  Pull up prefetch
on/off to start with, as there's only a single bit name different.

Pull up the definitions of common API functions too, starting with
flash_set_ws.  Even if the implementations are different, things that
meant to be the same, should be defined centrally.

lib/stm32/l4/Makefile

@@ -47,6 +47,7 @@ OBJS            += exti_common_all.o
 OBJS            += adc_common_v2.o adc_common_v2_multi.o
 OBJS            += crc_common_all.o crc_v2.o
 OBJS            += crs_common_all.o
+OBJS            += flash_common_all.o
 OBJS            += rng_common_v1.o
 OBJS            += timer_common_all.o
 OBJS            += i2c_common_v2.o