created SDdatalogger example

examples/SDdatalogger/boot.py

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+# boot.py -- runs on boot-up
+# Let's you choose which script to run.
+# > To run 'datalogger.py':
+#       * press reset and do nothing else
+# > To run 'cardreader.py':
+#       * press reset
+#       * press user switch and hold until orange LED goes out
+
+import pyb
+
+pyb.LED(3).on()
+pyb.delay(2000)
+pyb.LED(4).on()
+pyb.LED(3).off()
+switch = pyb.Switch()                   # check if switch was pressed decision phase
+
+if switch():
+    pyb.usb_mode('CDC+MSC')
+    pyb.main('cardreader.py')           # if switch was pressed, run this
+else:
+    pyb.usb_mode('CDC+HID')
+    pyb.main('datalogger.py')           # if switch wasn't pressed, run this
+
+pyb.LED(4).off()