Changes to the Adacore Libre GNAT GPL Ravenscar profile RTS for STM32F4
-----------------------------------------------------------------------

1. Imported Ravenscar SFP (Small Footprint Profile) RTS source files
   from AdaCore Libre GNAT GPL 2015 for STM32F4.

2. Changed the console serial port from USART1 to USART2 on pins
   PA2 and PA3, for compatibility with my C language framework.

3. Renamed s-bbpara.ads to s-bbpara.ads.original, and setup_pll.adb to
   setup_pll.adb.original.

4. Added customized s-bbpara.ads and setup_pll.adb for the
   STMicroelectronics NUCLEO-F411RE board.

5. Added customized memory map linker script for the NUCLEO-F411RE.

6. Removed previous memory map linker scripts for other boards.

7. Added support for the CERB40, NETDUINOPLUS2, STM32F4_DISCOVERY
   and STM32_M4_NINI boards.