The file BoardConfig.c is just the abstraction layer / interface to the HAL or BSP drivers of the Discovery board.
The framebuffer memory is reserved within the file main.c - there you can find the defines for FRAME_BUFFER_ADDR and FRAME_BUFFER_SIZE. The profile setting ScreenSize should always correspond to the real framebuffer size - otherwise you will get an error during the intialization.
For more details, please have a look to the article Platform Integration and Build Environments: Main Loop. It describes many details about initialization and integration.
In order to store resources within the QSPI flash memroy, please use the provided 'Template' project and set the flag QSPI_FLASH to 1 within the makefile of the project.
Please note: This flag determines whether the resources are stored within the code memory or separate within the QSPI flash memory.
Within your profile settings, you can determine how the resources are used during runtime:
The attributes FormatOfBitmapResources and FormatOfStringConstants can be set to 'DirectAccess' in case that the resources should be taken directly from flash memory. By default these attributes are set to 'Compressed'.