If boot sequencer is used with eSPI FLASH, can I enable it after boot sequencing is over in P1021? If I place config in eSPI FLASH, will it just overwrite whatever boot sequencer has done?
Boot sequencer serves a different purpose. It runs before the core starts. Booting from an eSPI flash, the core has to be configured correctly and starts the Boot-ROM code on-chip. It runs after the boot sequencer if any. So you can enable eSPI FLASH if boot sequencer has done all the necessary configurations. Also, the configurations in an eSPI FLASH will overwrite any memory mapped registers.
I want to run P1021 SPI in "SPI slave" mode. How should I configure SPI_SEL function for QE pin PB20?
When you configure pins CPPARBx[SELn]=11 and CPDIRxB[DIRn] = 11, it will configure PB20 as SPI_SEL function.