E407/: set HSE_VALUE to the clock frequency (else an incorrect default is used)
E407/serial.c (serial_init): increase data rate to 115200 bps

For compatibility with what we use under Contiki. 115.2 kbps has always
been the goal. The 38400 bps (effectively only about 18432 bps) detour
was caused by the clock calculations being wrong, but this is fixed now.
E407/demo.c (main): print the various clocks