libupm: enforce disabling python2

If there are more than two python3 interpreters, and no python2,
libupm thinks one of them is python2 and fails.  Avoid that by
telling it that python2 was not found.

Signed-off-by: Eneas U de Queiroz <cotequeiroz@gmail.com>

libs/libupm/Makefile

@@ -9,7 +9,7 @@ include $(TOPDIR)/rules.mk
 
 PKG_NAME:=libupm
 PKG_VERSION:=2.0.0
-PKG_RELEASE:=3
+PKG_RELEASE:=4
 
 PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tar.gz
 PKG_SOURCE_URL:=https://codeload.github.com/intel-iot-devkit/upm/tar.gz/v$(PKG_VERSION)?
@@ -53,7 +53,9 @@ UPM_MODULES:= \
 # (require libbacnet) tb7300 t8100 e50hx bacnetmstp
 # (require libtinyb) 2jciebu01_ble 2jciebu01_usb
 
-CMAKE_OPTIONS=-DBUILDSWIGNODE=$(if $(CONFIG_PACKAGE_libmraa-node),ON,OFF)
+CMAKE_OPTIONS=-DBUILDSWIGNODE=$(if $(CONFIG_PACKAGE_libmraa-node),ON,OFF) \
+	-DPYTHON2LIBS_FOUND=FALSE \
+	-DPYTHON2INTERP_FOUND=FALSE \
 
 define Package/libupm/Default
   SECTION:=libs