build: fix dpdk mellanox driver build setting

- dpdk mellanox driver setting requires rdma-core.mk is included
  before dpdk.mk makefile.  Include check for rdma-core_version
  variable definition in dpdk.mk to prevent a re-occurance of this
  issue.

Type: fix
Fixes: 70522a1dc

Change-Id: I297cbd734ac7abe970eac7fb317d06b7acfc5a40
Signed-off-by: Dave Wallace <dwallacelf@gmail.com>
diff --git a/build/external/Makefile b/build/external/Makefile
index d35c655..b4a099c 100644
--- a/build/external/Makefile
+++ b/build/external/Makefile
@@ -49,12 +49,12 @@
 include packages.mk
 include packages/ipsec-mb.mk
 include packages/quicly.mk
-include packages/dpdk.mk
 ifneq ($(shell uname), FreeBSD)
 include packages/rdma-core.mk
 include packages/xdp-tools.mk
 include packages/octeon-roc.mk
 endif	# ! FreeBSD
+include packages/dpdk.mk
 
 .PHONY: clean
 clean: