tcpdump, libpcap: simplify and fix ac_cv_linux_vers

tcpdump, libpcap: simplify and fix ac_cv_linux_vers

For some reason, tcpdump and libpcap need to have some information
about the kernel version being used. This information is passed using
the ac_cv_linux_vers autoconf variable.

However, the current value is determined using
BR2_DEFAULT_KERNEL_HEADERS which is only defined when an internal
Buildroot toolchain is used. So it would break with an external
toolchain or the Crosstool-NG backend.

According to Mike Frysinger at
http://lists.busybox.net/pipermail/buildroot/2011-January/040861.html,
this value is only used to determine if the kernel version is 0.x, 1.x
or 2.x, so passing ac_cv_linux_vers=2 is sufficient since Buildroot
only supports the 2.6 kernel anyway.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>