build: backport dpdk usertools support python3 only patch

build: backport dpdk usertools support python3 only patch

- On Centos-8, 'make install-ext-deps' fails due to the
  bpr-mangle-shebangs script failing the dpdk usertools
  python scripts containing '#!/usr/bin/env python'.
  Backport usertools section of the DPDK patch which
  fixes this issue:
  http://git.dpdk.org/dpdk/commit/?id=3f6f83626cf4967a99382a6518a614a1bf3d2c20

- Also fix README to reflect name change of dpdk master
  branch to 'main'.

Type: fix

Change-Id: I487b1ff2da786a4a3fd8fb0f859436b0e1885f1b
Signed-off-by: Dave Wallace <dwallacelf@gmail.com>