and restore a submodule that should have remained

Author: jepler

.gitmodules

@@ -92,6 +92,9 @@
 [submodule "frozen/circuitpython-stage"]
 	path = frozen/circuitpython-stage
 	url = https://github.com/python-ugame/circuitpython-stage.git
+[submodule "ports/cxd56/spresense-exported-sdk"]
+	path = ports/cxd56/spresense-exported-sdk
+	url = https://github.com/sonydevworld/spresense-exported-sdk.git
 [submodule "frozen/Adafruit_CircuitPython_SD"]
 	path = frozen/Adafruit_CircuitPython_SD
 	url = https://github.com/adafruit/Adafruit_CircuitPython_SD.git