build: add env variable to pass extra cmake args

If VPP_EXTRA_CMAKE_ARGS is set, its content will be
appended to the vpp cmake command cli

Type: feature

Change-Id: I825d4239e62b0a2fb70a652f0671f6c559630aad
Signed-off-by: Nathan Skrzypczak <nathan.skrzypczak@gmail.com>
(cherry picked from commit 2973654033)

build-data/packages/vpp.mk

@@ -42,6 +42,10 @@ ifneq ($(wildcard /opt/rh/devtoolset-7/enable),)
 vpp_cmake_args += -DCMAKE_PROGRAM_PATH:PATH="/opt/rh/devtoolset-7/root/bin"
 endif
 
+ifneq ($(VPP_EXTRA_CMAKE_ARGS),)
+vpp_cmake_args += $(VPP_EXTRA_CMAKE_ARGS)
+endif
+
 vpp_configure_depend += external-install
 vpp_configure = \
   cd $(PACKAGE_BUILD_DIR) && \