[tools] add bc as required by kernels 3.9+

 - bc is required by the kernel to compute timeconsts files
 - fixes #13767.

Signed-off-by: Zoltan HERPAI <wigyori@uid0.hu>



git-svn-id: svn://svn.openwrt.org/openwrt/trunk@37197 3c298f89-4303-0410-b956-a3cf2f4a3e73