esptool_py: add missing phony build targets for CMake

Adds targets erase_flash and monitor for users not using idf.py.
Closes https://github.com/espressif/esp-idf/issues/2420.

components/esptool_py/run_cmd.cmake

@@ -0,0 +1,15 @@
+if(NOT IDF_PATH)
+    message(FATAL_ERROR "IDF_PATH not set.")
+endif()
+include("${IDF_PATH}/tools/cmake/utilities.cmake")
+spaces2list(CMD)
+
+execute_process(COMMAND ${CMD}
+    WORKING_DIRECTORY "${WORKING_DIRECTORY}"
+    RESULT_VARIABLE result
+    )
+
+if(${result})
+    # No way to have CMake silently fail, unfortunately
+    message(FATAL_ERROR "${TOOL} failed")
+endif()