Fix CFG_NO_FLASH compilation.

Many Atmel boards have no "real" (NOR) flash on board, and rely only
on DataFlash and NAND memories. This patch enables CFG_NO_FLASH to
be present in a board configuration file, while still enabling flash
commands like 'flinfo', 'protect', etc.

Signed-off-by: Stelian Pop <stelian@popies.net>