For the SPI
* We improve the return semantics of _read() and _write()
* We set speed based on the value returned from ti_lib_sys_ctrl_clock_get() instead of using a hard-coded value
External flash changes:
* Rename macros to match instruction names
* verify_part(): Return a different value when the device is powered down and when communication fails
* Change return value semantics of static functions
* Adjust checks of board_spi_ return values
* Wait for BUSY to clear before attempting to send PD
* Accept two possible flash parts: W25X40CL (4MBit) as well as the W25X20CL (2MBit)
