From the app note:
I see that some processors have a flow control bit when booting in SPI slave mode.
The 21489 is not mentioned in the app-note, possibly because the app note came out before the 214xx series.
The earlier processors had only a 2-word fifo so flow control was necessary while the boot kernel was doing something that might take some time.
Does the 21489 also need flow-control in spi slave boot mode?