Kinetis:Use BOARD_xxxx to drive system clocking

   1) Shifted the clock speed of MK20DX128VLH5 to 48 Mhz to be
      able to uses USB.
   2) Set BOARD_OUTDIV3 to 0 - there is no BOARD_OUTDIV3 on a
      MK20DX128VLH5 or K20DX256VLH7

   3) Added BOARD_SOPT2_PLLFLLSEL and BOARD_SOPT2_FREQ along with
      settings for BOARD_SIM_CLKDIV2_USBFRAC and
      BOARD_SIM_CLKDIV2_USBDIV base on the BOARD_SOPT2_FREQ.