package/net-tools: use TARGET_CONFIGURE_OPTS to provide AR, CC and LDFLAGS

package/net-tools: use TARGET_CONFIGURE_OPTS to provide AR, CC and LDFLAGS

As noticed by Peter [1], use TARGET_CONFIGURE_OPTS to provide AR, CC and LDFLAGS.

Also remove TARGET_MAKE_ENV since it only contains PATH=$(BR_PATH) which is
also provided by TARGET_CONFIGURE_OPTS.

[1] http://lists.busybox.net/pipermail/buildroot/2016-May/162418.html

Cc: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>