chore(core): refactore trustzone initialization

[cepetr]

This pull request focuses on implementing a minimalistic initialization of TrustZone on the STM32U5 in the board loader:

1. The SAU (Security Attribution Unit) remains uninitialized, ensuring that all memory space is secured
2. The FPU is enabled in both secured and unsecured modes
3. FLASH controller per-page settings are utilized to configure all flash pages as secured and unprivileged
4. GTZC MPCBB block and sets all SRAM blocks are configured as secured and unprivileged
5. All peripherals are set to be secured & unprivileged

Note: The option bytes for the Flash secure watermark have been modified. As a result, only the first 64KB of FLASH memory is initially marked as secure after a reboot. Subsequently, in step 3 of the initialization process, the remaining FLASH memory is designated as secure.

[TychoVrahe]

LGTM, just few minor nits