Kinetis:Extend clockconfig to support SOPT2_PLLFLLSEL and SIM_CLKDIV3

  A board.h file can now specify the:
   1) BOARD_SOPT2_PLLFLLSEL to select the output of the SIM_SOPT2 MUX
     from:
       MCGFLLCLK
       MCGPLLCLK
       USB1PFD
       IRC48MHZ

    2) If it defines BOARD_SIM_CLKDIV3_FREQ then it must define
    BOARD_SIM_CLKDIV3_PLLFLLFRAC and BOARD_SIM_CLKDIV3_PLLFLLDIV
    which wil be used to cpnfigure SIM_CLKDIV3 [PLLFLLFRAC, PLLFLLDIV]