iproute2: fix no-thread build

iproute2: fix no-thread build

iproute2 believes that it needs to link with libpthread for its arpd
binary, because "some db implementations require thread". Therefore, our
iproute2.mk explicitly disables the build of arpd when thread support is
not available.

However, the sed expression it uses no longer works. The Makefile used
to look like:

TARGETS = foo baz baz arpd foobar

so replacing " arpd " with a space was working fine. However, the
Makefile got changed in iproute2 to:

ifeq (... berkeleydb available ...)
     TARGETS += arpd
endif

i.e, with no space at the end of the line. This made our sed expression
ineffective, causing build issues with no-thread configurations since
arpd was no longer disabled.

To address this, instead of sed-ing the Makefile, we overwrite the
berkeleydb detection of iproute2, by writing to the "Config" file, like
we're doing for other aspects of the package.

Fixes:

  http://autobuild.buildroot.net/results/03a37a2372a4c2e438a073e015c49d9e554b86b7/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>