Updated CMake for ESP-IDF (#748)

CMakeLists.txt

@@ -1,5 +1,25 @@
 cmake_minimum_required(VERSION 3.13)
 
+if(ESP_PLATFORM)
+	# Build RadioLib as an ESP-IDF component
+  # required because ESP-IDF runs cmake in script mode
+  # and needs idf_component_register()
+  file(GLOB_RECURSE RADIOLIB_ESP_SOURCES
+    "src/*.*"
+  )
+
+	idf_component_register(
+    SRCS ${RADIOLIB_ESP_SOURCES} 
+    INCLUDE_DIRS . src 
+  )
+
+	return()
+endif()
+
+if(CMAKE_SCRIPT_MODE_FILE)
+  message(FATAL_ERROR "Attempted to build RadioLib in script mode")
+endif()
+
 project(radiolib)
 
 file(GLOB_RECURSE RADIOLIB_SOURCES
@@ -23,4 +43,3 @@ install(DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/src/
   DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/RadioLib
   FILES_MATCHING PATTERN "*.h"
 )
-