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
* Erase range operations allow preemption after each block or sector.
* Write operations allow preemption every 8KB of data.
* Reado operations allow preemption every 16KB of data.
Without this, it's possible for stale information to be read from
cache via mmap, even if the MMU table entry had been invalidated
prior to writing flash (if the same MMU table entry was re-used after
writing flash.)
