iproute2: fix build with threads support disabled

iproute2: fix build with threads support disabled

iproute2 renamed the configure script generated file to config.mk.
Update the berkeleydb disable code.

Fixes:
http://autobuild.buildroot.net/results/46d/46da1142806fb524326994585cec6de9f7f3f480/
http://autobuild.buildroot.net/results/3aa/3aa028fe9e730335d2af00d5969b674a3d9f640b/

Cc: Petr Vorel <petr.vorel@gmail.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Acked-by: Petr Vorel <petr.vorel@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>