buildroot

mongrel2: allow to build for sparc

With uClibc-ng 1.0.15 sparc support got context functions.
Tested with qemu-system-sparc.

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