Err, I guess a devel board from the SoC vendor? Then later a series of prototypes for the phone/tablet.
If you meant to ask about the bootloader, I think each vendor has their own. They do seem to have similarities though, so I wouldn't be surprised if they are derived from something originally written by Google and/or ARM.
("hardware bring-up" is a specific term, Google it)
Yeah I wasn't referring to the hardware devel stages :P I was more asking what is the typical android phone's version of BIOS/(U)EFI, if it even had one at all?