mongrel2: Needs a {e, }glibc or a uClibc-0.9.34-git based toolchain

mongrel2 needs the {get,make,swap}context functions which are
present in {e,}glibc and in the uClibc's master branch.

Fixes:
http://autobuild.buildroot.net/results/8e132a2b03a7b5ae94199db7db86883c05dca700/build-end.log



Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>