common.mk: add PYTHON3 variable

Add PYTHON3 variable and replace python calls in common makefiles and in
trusted-services.mk. This way the Python interpreter can be selected
externally.

Signed-off-by: Imre Kis <imre.kis@arm.com>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>
diff --git a/trusted-services.mk b/trusted-services.mk
index 2bac479..64fc988 100644
--- a/trusted-services.mk
+++ b/trusted-services.mk
@@ -86,7 +86,7 @@
 OPTEE_OS_COMMON_EXTRA_FLAGS += SP_PATHS="$(optee_os_sp_paths)"
 else ifeq ($(SP_PACKAGING_METHOD),fip)
 $(TS_INSTALL_PREFIX)/sp_layout.json: ffa-sp-all
-	python $(TS_PATH)/tools/python/merge_json.py $@ $(fip_sp_json_paths)
+	$(PYTHON3) $(TS_PATH)/tools/python/merge_json.py $@ $(fip_sp_json_paths)
 
 optee-os-common: $(TS_INSTALL_PREFIX)/sp_layout.json