[BUILD] Add support for openocd helpers with cmake

- Add openocd_<target>: runs openocd for the target
- Add openocd_flash_<target>: flashes the target using openocd.
 It works with both an already running openocd instance as well
 as launching its own one. Uses a helper tool that requires python.
- Add openocd_cfg_<target>: generates openocd config for target. Used
 for generating an openocd config automatically when launching
 a debug session from an IDE.

cmake/stm32f4.cmake

@@ -76,7 +76,7 @@ set(STM32F4_DEFINITIONS
 )
 
 function(target_stm32f4xx name startup ldscript)
-    target_stm32(${name} ${startup} ${ldscript} ${ARGN})
+    target_stm32(${name} ${startup} ${ldscript} OPENOCD_TARGET stm32f4x ${ARGN})
     if (IS_RELEASE_BUILD)
         target_compile_options(${name} PRIVATE "-O2")
         target_link_options(${name} PRIVATE "-O2")