mimxrt/boards: Support using an optional board-specific manifest.py.

If the board directory contains a manifest.py file, it will be included.
File not found errors will be ignored.

ports/mimxrt/boards/manifest.py

@@ -2,3 +2,7 @@ freeze("$(PORT_DIR)/modules")
 freeze("$(MPY_DIR)/drivers/onewire")
 freeze("$(MPY_DIR)/drivers/dht", "dht.py")
 include("$(MPY_DIR)/extmod/uasyncio/manifest.py")
+try:
+    include("$(BOARD_DIR)/manifest.py")
+except FileNotFoundError:
+    pass