[Buildroot] Improved patches for Arietta G25 and Aria G25 boards

[Biagio Montaruli]

Hi all,
I have improved the previous patches that I sent to improve Buildroot
configuration for Arietta G25 and Aria G25.
Here you are the main changes :

1) Use the mainline linux kernel (the default version used by Buildrooot)

2) Add post-build.sh script beacause, the build of AT91bootstrap produces
   "at91sam9x5_arietta-sdcardboot-linux-zimage-dt-3.8.6.bin" that needs to be
   renamed to "boot.bin" because the boot ROM code expects to load a file named
   ar91bootstrap3 and moves the binary files in this directory.
   Finally the script create a copy of at91sam9x5_arietta-sdcardboot-linux-zima$
   with the boot.bin name that is used to build the sdcard.img with genimage.
   It is necessary to move the boot.bin link in another directory since
   we can't rename at91sam9x5_arietta-sdcardboot-linux-zimage-dt-3.8.6.bin
   to boot.bin because at91sam9x5_arietta-sdcardboot-linux-zimage-dt-3.8.6.bin
   and the boot.bin link are in the same directory (to avoid name collision).
   Regarding the AT91bootstrap package in Buildroot,
   I think that it should be updated to 3.8.6 version since Buildroot
   uses the 3.8 version

3) Add genimage.cfg in order to build sdcard.img

4) Remove the local configuration of usb0
   (which was added in the previous series of patches I sent)

5) Update documentation in readme.txt file for both Arietta and Aria

Best regards,
Biagio