Support python3 only systems

Ubuntu 20.04 and latest Cygwin come with no Python 2 and no link
from python to python3. To not mess with the system we detect
python3 for seamless support.

platforms/nuttx/NuttX/tools/kconfig-conf

@@ -6,8 +6,14 @@ DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" >/dev/null 2>&1 && pwd )"
 export APPSDIR="`pwd`/../apps"
 export CONFIG_ARCH_BOARD_CUSTOM=y
 
+if [ "command -v python3" ]; then
+	PYTHON_EXECUTABLE=python3
+else
+	PYTHON_EXECUTABLE=python
+fi
+
 if [ "${1}" = "--olddefconfig" ]; then
-	PYTHONPATH=${DIR} python ${DIR}/olddefconfig.py > /dev/null
+	PYTHONPATH=${DIR} ${PYTHON_EXECUTABLE} ${DIR}/olddefconfig.py > /dev/null
 else
 	echo "ERROR: ${@} unsupported"
 	exit 1

src/drivers/uavcan/CMakeLists.txt

@@ -93,7 +93,7 @@ foreach(DSDLC_INPUT ${DSDLC_INPUTS})
 	list(APPEND DSDLC_INPUT_FILES ${DSDLC_NEW_INPUT_FILES})
 endforeach(DSDLC_INPUT)
 add_custom_command(OUTPUT px4_uavcan_dsdlc_run.stamp
-	COMMAND ${PYTHON} ${LIBUAVCAN_DIR}/libuavcan/dsdl_compiler/libuavcan_dsdlc ${DSDLC_INPUTS} -O${DSDLC_OUTPUT}
+	COMMAND ${PYTHON_EXECUTABLE} ${LIBUAVCAN_DIR}/libuavcan/dsdl_compiler/libuavcan_dsdlc ${DSDLC_INPUTS} -O${DSDLC_OUTPUT}
 	COMMAND ${CMAKE_COMMAND} -E touch px4_uavcan_dsdlc_run.stamp
 	DEPENDS ${DSDLC_INPUT_FILES}
 	COMMENT "PX4 UAVCAN dsdl compiler"