env: dataflash: fix env_init issue

As the SPI controller is not initialized before env_init(), it causes
reading env in dataflash failed. So, although saveenv() successfully,
it shows warning information when reboot the system as following:

  *** Warning - bad CRC, using default environment

Let the env_relocate() to check env CRC and import it.

Signed-off-by: Bo Shen <voice.shen@atmel.com>