package/iproute2: add optional support for elfutils

package/iproute2: add optional support for elfutils

When elfutils was compiled before, iproute2 will use it as optional
dependency:

$ output/host/usr/bin/x86_64-linux-readelf -a output/target/sbin/tc | grep NEEDED
 0x0000000000000001 (NEEDED)             Shared library: [libelf.so.1]
[...]

The build system offers no option to en-/disable elfutils support:
http://git.kernel.org/cgit/linux/kernel/git/shemminger/iproute2.git/tree/configure#n252

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>