E407/system_stm32f4xx.c: copy of .../CMSIS/Device/ST/STM32F4xx/Source/Templates/system_stm32f4xx.c
E407/Makefile.stm32 (PATH_SYSTEM_C): use local system_stm32f4xx.c
E407/system_stm32f4xx.c: adjust PLL settings to (finally !) yield 168 MHz