netperf_udp test: Use utils_netperf downloading

utils_netperf has code for downloading the netperf source, should the
input path be an URL. Remove the local function for doing this, and let
utils_netperf handle it instead.

Signed-off-by: Jonas Eriksson <jonas.eriksson@enea.com>
Remove CC= from Makefiles

CC= assignments makes it impossible to supply alternate CC:s from the
command line. Additionally, CC?= does not work, as CC is set to 'cc' by
default. A possible solution might be:

ifeq ($(origin CC), default)
  CC=gcc
endif

, but seeing as cc is the preferred compiler on the target, the CC
assignment seemed unneccessary in either case.

Signed-off-by: Jonas Eriksson <jonas.eriksson@enea.com>