buildroot

mono: disable for sparc/sparc64

It seems mono isn't supported for sparc32, see configure.ac
for some comments about it. Tried with sparc64, but this
doesn't compile either.
So disable for sparc for now.

Fixes following autobuild issues:
http://autobuild.buildroot.org/?reason=mono-4.2.2.30

Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>