1) fixed SPI_read_status: added check for flash busy flag in matrix mode
2) fixed SPI_page_program: enable write before writing data to SPI FIFO
3) SPI flash ROM funcs replacement is controlled via menuconfig option
Also add steps to disable flash encryption, as some people seem to
accidentally enable it.
Explicitly mark the flash encryption and secure boot as "READ DOCS
FIRST" in menuconfig.
* App access functions are all flash encryption-aware
* Documentation for flash encryption
* Partition read/write is flash aware
* New encrypted write function
