New config directive USE_FLASH_FAR_DESCRIPTORS to enable FLASH descriptors anywhere, i.e. read memory pgm_*_far fuctions. It's not needed for main program but for large chip bootloaders when RAMPZ register is needed to access bootloader's FLASH. Currently USE_RAM_DESCRIPTORS must be used which allocates sligtly more RAM for consts. It's rather minor problem but I got it when developing bootloader for AT90USB1286.
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