arduino/examples: readback_test uses new API

The DuinoCube API was updated in commit:
35dde88d8656dbbe2a353e4ee71ffd2eb984a8f3
arduino: include core defs in core include

This is a folow-up to the change in commit:
35dde88d8656dbbe2a353e4ee71ffd2eb984a8f3
duinocube: Switch core SPI code to MSB first mode

This fix goes with commit:
2713c84d5457894d3376505e03e6a9c94c24e73f
duinocube: Update SPI memory access protocol

Need to first send a byte code indicating it is a memory access.
arduino/firmware: file_read uses file handle correctly
arduino/firmware: Put strings in progmem

This saves RAM by loading strings into RAM only when they're used.
duinocube: RPC funcs for seek and size
duinocube: Add API for reading from file to Core directly