ULX3S: Confirm working on boards with ISSI flash

[dan-rodrigues]

The current flash configuration on ULX3S is known to work on the blue boards with Winbond flash but not the older boards with ISSI flash. Some vendor specific commands are used in the boot code and there are differences between vendors. The "XIP" command byte is also different.

This recent merge is thought to make it work with ISSI flash too. It will read the vendor ID and print it on screen before jumping to user program. This is confirmed working on a Winbond board but needs testing on an ISSI one. If it fails on the ISSI board, it will either print "ISSI" and freeze on screen or print "UNRECOGNIZED FLASH VENDOR" if it didn't get any expected vendor ID.