Moved custom defaults to dedicated block for STM32F745.

[mikeller]

Rationale: Makes it easier to debug with STM32F745 targets. Currently there is a lot of unused flash on this target, so we may as well use it to make debugging easier (i.e. flashing with make STM32F745_flash without losing custom defaults).

Before:

AXIM_FLASH_CUSTOM_DEFAULTS:           8 B        22 KB      0.04%
AXIM_FLASH_CONFIG:          0 GB        32 KB      0.00%
     AXIM_FLASH1:      503864 B       960 KB     51.26%
AXIM_FLASH_CUSTOM_DEFAULTS_EXTENDED:          0 GB         0 GB      -nan%

After:

AXIM_FLASH_CUSTOM_DEFAULTS:           8 B        22 KB      0.04%
AXIM_FLASH_CONFIG:          0 GB        32 KB      0.00%
     AXIM_FLASH1:      503864 B       928 KB     53.02%
AXIM_FLASH_CUSTOM_DEFAULTS_EXTENDED:          0 GB        32 KB      0.00%