allow mkimage to build and run on Cygwin-hosted systems

* Patch by Frank Mller, 18 Sep 2003:
  use bi_intfreq instead of bi_busfreq to compute fec_mii_speed in
  cpu/mpc8xx/fec.c

* Patch by Pantelis Antoniou, 16 Sep 2003:
  add tool to compute fileds in the PLPRCR register for MPC86x