Fix CMake Board Path's Command (#2309)

The Renesas manifest file is not in the root of the vendor directory, and causes an error when earching for vendor paths.

To reproduce:
Before merging this commit
` cmake -P tools/cmake/afr_board_paths.cmake`

Expected Output:
cmake_supported_boards.yml created

Actual Output:
CMake Error at tools/cmake/afr_board_paths.cmake:19 (include):
  include could not find load file:

/Users/lundinc/Documents/amazon-freertos/vendors/renesas/manifest.cmake

vendors/renesas/rx_mcu_boards/manifest.cmake → vendors/renesas/manifest.cmake

@@ -4,4 +4,4 @@ set(
     CACHE INTERNAL "Supported boards list."
 )
 
-set(AFR_MANIFEST_BOARD_DIR "boards")
+set(AFR_MANIFEST_BOARD_DIR "rx_mcu_boards/boards")