orc: disable for uclinux

Fixes http://autobuild.buildroot.net/results/445/4456e49a6c33a80570b4f59ec9799a8d4a314af1/



The orc configure script doesn't recognize uclinux, so builds using
BR2_BINFMT_FLAT fails.  While this could be patched, orc doesn't have any
acceleration for blackfin and orc isn't a hard dependency of any other
package, so just disable it.

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