arm: atmel: sam9m10g45ek: let CONFIG_SYS_NO_FLASH at 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, if the
CONFIG_SYS_NO_FLASH defined later than include/config_cmd_default.h,
These two commands will be included always.

So move CONFIG_SYS_NO_FLASH definition to proper position.

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