rapidxml: new package

[Thomas:
  - Rewrap Config.in help text
  - Indicate that the SHA256 hash has been locally calculated, and add
    the MD5 and SHA1 hashes from SourceForge.
  - Use downloads.sourceforge.net as the download site instead of
    garr.dl.sourceforge.net, in order to use the SourceForge mirrors
    properly, and be consistent with all other Buildroot packages
    downloading stuff from SourceForge.
  - Change the license from "MIT" to "Boost Software License 1.0 or
    MIT" as indicated by the license.txt file.
  - Change the <pkg>_EXTRACT_CMDS to remove the sub-directory created
    by the .zip file extraction, and simplify the staging installation
    step accordingly.]

Signed-off-by: Jonathan Ben Avraham <yba@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>