rpi-userland: not available for static only builds

Fixes:
http://autobuild.buildroot.net/results/e9e/e9ea569b5fe6fd875b3fa8f4eee82e3fddd95e90/
http://autobuild.buildroot.net/results/c97/c97f7df043469bc51ebbb41ddf30f8e82d4e6bb5/
http://autobuild.buildroot.net/results/ba8/ba82769109f205123834627bbf782bd1811ea43e/



Signed-off-by: Peter Korsgaard <peter@korsgaard.com>