First of all, I know about this:
This is true, in case of Raspberry Pi we can't boot without SD card. For most users SD card is ideal boot source I think.How does it boot?
You have to boot from SD but a USB HD can “take over” after the initial boot. You cannot boot without an SD card.
I have to create my custom PCB board based on BCM2835 which is 99.9% same as RPi, because I need more robust hardware design to be vibration tolerant (and also some other task-specific modification).
AFAIK SDIO interface is same as SPI interface, so I suppose I can use small SPI Flash/DataFlash IC chip as boot source. Could I do so?
Yes, flashing of memory IC in such case is a some sort of pain but it's acceptable. I hope I can use some jumpers to flash it outside.
Unfortunately I can't learn about this from available BCM2835-ARM-Peripherals.pdf datasheet, it looks like very cut and incomplete in comparison to other manufacturers.