scripts/functions: add aria2, a powerfull downloader

aria2 is a powerfull downloader that is capable of chunking and
parallel retrieval.

Due to li;itations in crosstool-NG retrieval facilities, it's not possible
to take fully advantage of aria2. It might happen that, in the future,
those limitations get lifted away, so we can take use features such as
parallel downloading from more than one server at the same time. For now,
it should still speed up downloads thanks to parallel downloading of chunks.

# x86 specific options

config ARCH_x86

    select ARCH_SUPPORTS_32

    select ARCH_SUPPORTS_64

    select ARCH_DEFAULT_32

    select ARCH_USE_MMU

    select ARCH_SUPPORT_ARCH

    select ARCH_SUPPORT_CPU

    select ARCH_SUPPORT_TUNE

    help

      The x86 architecture, as defined by:

        32-bit (ia32) : http://www.intel.com/

        64-bit (amd64): http://www.amd.com/us-en/Processors/DevelopWithAMD/0,,30_2252_875_7044,00.html