prepare_port sel4: check required python modules

This prevents undescriptive errors when building sel4.

Ref #2490

repos/base-sel4/ports/sel4.hash

@@ -1 +1 @@
-112234357bb0cd71d9401f183734fa5b784543e1
+8ab77c39000530e8f78a66581bf368a902cbcdc3

repos/base-sel4/ports/sel4.port

@@ -8,6 +8,9 @@ REV(sel4) := 8564ace4dfb622ec69e0f7d762ebfbc8552ec918
 DIR(sel4) := src/kernel/sel4
 
 $(call check_tool,python)
+$(call check_python_module,future)
+$(call check_python_module,tempita)
+$(call check_python_module,ply)
 
 PATCHES   := $(wildcard $(REP_DIR)/patches/*.patch)
 

tool/ports/mk/install.mk

@@ -25,6 +25,11 @@ STRICT_HASH ?= no
 #
 check_tool = $(if $(shell which $(1)),,$(error Need to have '$(1)' installed.))
 
+#
+# Utility to check if a python module is installed
+#
+check_python_module = $(if $(shell python -c "import $(1)" 2>&1),$(error Need to have python module '$(1)' installed.),)
+
 default:
 
 .NOTPARALLEL: default