With the old build system in RTEMS 5 that was possible by just
overwriting BOARD_Sdram_Config and setting a custom
ATSAM_MEMORY_SDRAM_SIZE during building the BSP. In the new build system
that ATSAM_MEMORY_SDRAM_SIZE is set exclusively by the selected SDRAM
chip.
This patch adds the possibility to specify a "custom-0x100000" or
similar as SDRAM type where the number gives the SDRAM size.
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