configs/stm32f429_disco: new configuration for STM32F429 Discovery board

configs/stm32f429_disco: new configuration for STM32F429 Discovery board

This commit adds a defconfig for the STM32F429 platform, which is
based on a Cortex-M4 core from ST Microelectronics. It is therefore
the first noMMU ARM platform supported in Buildroot.

This commit includes some files that will be common to several STM32
platforms (hence in board/stmicroelectronics) and some files that are
specific to the STM32F429 (hence in
board/stmicroelectronics/stm32f429-disco). More specifically, this
commit adds:

 - A minimal Busybox configuration, which is small enough to boot
   without causing OOM on such small noMMU platforms. The resulting
   Busybox, statically linked with uClibc-ng, weights around 220
   KB. For now, this file is located in board/stmicroelectronics/, but
   we might consider moving it to package/busybox/ in the future if
   needed.

 - A post-build script that removes the mounting of /dev/pts (not
   enabled in the kernel and not very useful for a system that has no
   network and no X), and removes the network related init script and
   configuration files (no network support).

 - A flash.sh script, to perform the right OpenOCD invocations to
   reflash the board.

 - One small kernel patch to adjust the kernel command line in the
   Device Tree, since it's the only way to do so.

 - The usual readme.txt file.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
[Thomas:
 - squashed multiple patches from Lee Jones together
 - added the minimal Busybox configuration
 - added the post-build script
 - improved the flashing script to not hardcode the location of the
   output directory
 - add the small kernel patch
 - improve the readme.txt file
 - test on HW the resulting image, after using the internal toolchain.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>