package/ipmiutil: really force md2 off

If ALLOW_GNU is not defined, SKIP_MD2 is a nop.

Fixes:
http://autobuild.buildroot.net/results/f606ec1fff4cea257a4c6274bc5603efdfc439ad/



Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Cc: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>