ERASE_TIMEOUT was defined but never used; the final erase ACK used
the plain short WAIT_CYCLES budget, so a slow remote flash made
ext_flash_erase() fail even when the erase succeeded. Factor
wait_ack_cycles() out of wait_ack() and use WAIT_CYCLES *
ERASE_TIMEOUT for the erase ACK, like uart_rx_timeout() does with
READ_TIMEOUT. Add unit tests for the success and timeout paths.
- added img->hdr_ok flag when opening external flash image
- correct return value for uart_flash ext_flash functions
- correct image size in prepare_encrypte_update.sh