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The H6 has quite a large chunk of continuous SRAM, and also the BROM
allows to load eGON images far bigger than 32KB.

Move the FEL stack backup buffers and the thunk address towards the end
of SRAM C, so that we have a larger chunk of continuous SRAM available
for the SPL.
Signed-off-by: Andre Przywara <osp@andrep.de>