This is dynamically set by hw in system_mode_ctrl. ROM will reset to
executable, but will be marked as non-executable as soon as we are no
longer executing in ROM, like system_mode.
ROM will be marked as executable again, if function calls are made to
either `syscall_addr_reg` or `blake2s_addr_reg`. Set reset value of
`blake2s_addr_reg` to an illegal address, halting the CPU if it is
called unset.
The blake2s function is 4-byte aligned, to ensure the cpu_addr is is
aligned with the address in the register.
Co-authored-by: Mikael Ågren <mikael@tillitis.se>
