Makefile: Alter VPP_PYTHON_PREFIX for preloading deps

Allow setting of VPP_PYTHON_PREFIX to alternate location
so the python prereqs can be installed into base image
Also added test-dep trigger to isolate dependency install
from actual test run

Change-Id: Ia80f5dbf71bc24eb46cd6586bcadd474ef822704
Signed-off-by: Ed Kern <ejk@cisco.com>

Makefile

@@ -330,7 +330,7 @@ wipe-release: test-wipe $(BR)/.deps.ok
 
 rebuild-release: wipe-release build-release
 
-export VPP_PYTHON_PREFIX=$(BR)/python
+export VPP_PYTHON_PREFIX ?= $(BR)/python
 
 libexpand = $(subst $(subst ,, ),:,$(foreach lib,$(1),$(BR)/install-$(2)-native/vpp/$(lib)/$(3)))
 
@@ -377,6 +377,9 @@ test-shell:
 test-shell-debug:
 	$(call test,vpp,vpp_debug,shell)
 
+test-dep:
+	@make -C test test-dep
+
 test-doc:
 	@make -C test doc
 

test/Makefile

@@ -128,6 +128,8 @@ sanity: verify-no-running-vpp
 ext:
 	make -C ext
 
+test-dep: verify-python-path $(PAPI_INSTALL_DONE)
+
 test: verify-python-path $(PAPI_INSTALL_DONE) ext sanity reset
 	$(call retest-func)