Flash images for xula2-stickit4 boards

This is the as of March 2022 current flash image for Sancus and the xula2-stickit4 boards. The configurations are both for 128 bit security and are clocked at 20 and 8MHz respectively. The specific compilation of Sancus is without ATOMICITY_MONITOR, thus the pure Sancus configuration.
The default Sancus settings depend on 20MHz while some special situations may be more suited for the 8MHz configuration (like RIOT which can better make use of 8MHz for the internal clock). Ensure that you adjust the baudrates when using the 8MHz image. Where 20MHz requires a baudrate of 115200, 8MhZ requires a baudrate of 57600. Places where the baudrate needs to be adjusted:

1. In the sancus-support libraries if they are used. File https://github.com/sancus-tee/sancus-support/blob/master/src/dev/uart_hardware.h#L56 needs to use 57600 instead of the default 115200 baudrate (i.e., the line #define BAUD 174 needs to be changed to #define BAUD 139).
2. For sancus-loader, for example sancus-loader -device /dev/ttyUSB0 -baudrate 57600 main.elf
3. Monitor output of the FPGA, for example with screen screen /dev/ttyUSB1 57600