Enlarge bootloader flash area in (selected) BSP

mbedtls3 RSA is quite big and it no longer fits into typical 32k flash area. This is causing issues with latest mcuboot.

We should think on how this could be handled in generic and backward compatible way at least for BSPs that has enough flash.
Also should decide on new 'default' value 48k or 64k ?

possible options:
- new flash map conditional if RSA is being used in bootloader
- new flash map unconditional
- reuse reboot log are where possible (and make it mutually exclusive with RSA signing in mcuboot)

for reference:
https://github.com/mcu-tools/mcuboot/pull/2712

Start by reviewing the BSP flash-map handling and the referenced mcuboot pull request. Compare conditional and unconditional flash-map changes with reuse of the reboot-log area, including backward compatibility and BSP flash capacity. Done means an agreed generic approach and default flash-area size, either 48k or 64k, with affected BSPs identified.

Written by the indexing model from the issue text.