arm: atmel: at91sam9x5: move CONFIG_SYS_NO_FLASH to proper position

In config_cmd_default.h, it will use CONFIG_SYS_NO_FLASH to decide
whether include CONFIG_CMD_FLASH and CONFIG_CMD_IMLS. So, move the
CONFIG_SYS_NO_FLASH to proper position, then we don't need to undef
these two commands.

Signed-off-by: Bo Shen <voice.shen@atmel.com>
Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>