env: fix allow to build multiple environments

Patch fixes build error when enabling CONFIG_ENV_IS_IN_SPI_FLAS
and CONFIG_ENV_IS_IN_MMC at the same time mentioned issue in
below link:

Refer: https://lists.denx.de/pipermail/u-boot/2018-February/319565.html



build error when enabling CONFIG_ENV_IS_IN_SPI_FLASH and
CONFIG_ENV_IS_IN_MMC at the same time.

Signed-off-by: Rajesh Bhagat <rajesh.bhagat@nxp.com>
Reviewed-by: York Sun <york.sun@nxp.com>