doc: Note need of bc in the kernel build from 3.10 onwards

HPA did the world a favour and reduced the number of perl scripts in the
universe. However we do now need bc

Resolves-bug: https://bugzilla.kernel.org/show_bug.cgi?id=60575


Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>