According to section 4 of document AN-B-001 DA14580 Booting from Serial Interface, it seems that there is only one option to boot from SPI, which uses P0.0, P0.3, P0.5 and P0.6.
I do not konw if this is really the only option for SPI booting. And, will this option be problematic for downloading my progam into my SPI-Flash, if I use UART pins P0.0 and P0.1, and SmartSnippets Toolbox V5.0.6.2196?