mbed fix for Infinity
=====================
Without linker script patch it doesn't place vector table in final binary.
And clock is configured to 48MHz using internal clock reference and FLL multiplication.


mbed/targets/cmsis/TARGET_Freescale/TARGET_K20D50M/system_MK20D5.c
    Fix SystemInit: clock setup for internal clock. Inifinity has no external Xtal.

mbed/targets/cmsis/TARGET_Freescale/TARGET_K20D50M/cmsis_nvic.c
    Fix NVIC vector address of firmware 0x1000 instead of 0x0

mbed/targets/cmsis/TARGET_Freescale/TARGET_K20D50M/TOOLCHAIN_GCC_ARM/MK20D5.ld
    Fix memory map for Infinity bootloader
        Flash starts at 0x1000
        No flash config bytes sector

USBDevice/USBDevice/USBHAL_KL25Z.cpp
    Fix USB clock setup, see below.


2015/01/04  Based on mbed-sdk @2f63fa7d78a26.



Kinetis USB config
==================
Clock source: Internal reference clock wth FLL
    SIM_SOPT[USBSRC] = 1(MCGPLLCLK/MCGFLLCLK)
    SIM_SOPT[PLLSEL] = 0(MCGFLLCLK)

Clock dividor:
    SIM_CLKDIV2[USBDIV] = 0
    SIM_CLKDIV2[USBFAC] = 0

Clock enable:
    SIM_SCGC4[USBOTG] = 1


