Add xenomai real-time Framework to buildroot

[Fixes by Thomas, including comments from Arnout:
  * Use AUTOTARGETS instead of GENTARGETS

  * Use $(KERNEL_ARCH) instead of $(BR2_ARCH) as argument to the
    prepare-kernel.sh script. This allows the arch name to be fixed
    with the usual sed expressions and the quotes to be stripped.

  * Add the --verbose option to prepare-kernel.sh. This allows to get
    some clear error message when no Xenomai patch has been found for
    the current kernel version.

  * Improve the help texts as suggested by Arnout, and remove the
    now useless README file.

  * Add a BR2_PACKAGE_XENOMAI_SMP option, instead of poking inside the
    kernel configuration to find out whether SMP is enabled or
    not. This cannot work because: 1) the kernel might be built
    outside of Buildroot and 2) if the kernel is built inside
    Buildroot, it is built *after* Xenomai, so the kernel
    configuration file is typically not yet present.

  * Simplify the ARM subarchitecture selection as suggested by Arnout.

  * Remove the documentation and development files according to
    Buildroot standards (using BR2_HAVE_DOCUMENTATION and
    BR2_HAVE_DEVFILES).

  * Simplify the /etc/ld.so.conf modification logic.
]

[Peter: whitespace fixes]
Signed-off-by: Benoit Mauduit <benoit.mauduit@openwide.fr>
Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>