python,python3: allow users to override python args & vars

If users want to define these before including python[3]-package.mk, these
vars will be overridden during the include.
So, override these vars if they haven't been defined.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>

lang/python/python-package.mk

@@ -115,8 +115,8 @@ define Build/Compile/PyMod
 endef
 
 PYTHON_PKG_SETUP_DIR ?=
-PYTHON_PKG_SETUP_ARGS:=--single-version-externally-managed
-PYTHON_PKG_SETUP_VARS:=
+PYTHON_PKG_SETUP_ARGS ?= --single-version-externally-managed
+PYTHON_PKG_SETUP_VARS ?=
 
 define PyBuild/Compile/Default
 	$(foreach pkg,$(HOST_PYTHON_PACKAGE_BUILD_DEPENDS),

lang/python/python3-package.mk

@@ -114,8 +114,8 @@ define Build/Compile/Py3Mod
 endef
 
 PYTHON3_PKG_SETUP_DIR ?=
-PYTHON3_PKG_SETUP_ARGS:=--single-version-externally-managed
-PYTHON3_PKG_SETUP_VARS:=
+PYTHON3_PKG_SETUP_ARGS ?= --single-version-externally-managed
+PYTHON3_PKG_SETUP_VARS ?=
 
 define Py3Build/Compile/Default
 	$(foreach pkg,$(HOST_PYTHON3_PACKAGE_BUILD_DEPENDS),