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.

 # s390 specific config options

 # depends on EXPERIMENTAL

 config ARCH_s390

     select ARCH_SUPPORTS_32

     select ARCH_SUPPORTS_64

     select ARCH_DEFAULT_32

     select ARCH_USE_MMU

     select ARCH_SUPPORT_FPU

     help

       Defined by IBM in:

         32-bit ESA/390       : http://publibfp.boulder.ibm.com/cgi-bin/bookmgr/BOOKS/dz9ar008/CCONTENTS

         64-bit z/Architecure : http://publibfp.boulder.ibm.com/cgi-bin/bookmgr/BOOKS/dz9zr002/CCONTENTS