env: sunxi: enable ENV_IS_IN_SPI_FLASH

Now that sunxi uses CONFIG_SPI more sanely, and can also now properly
load the environment from SPI flash, let's enable the symbol that
actually considers the SPI flash when accessing the environment.

As this symbol depends on CONFIG_SPI, which we now only enable if the
board has a SPI flash, we can make if "default y" for all Allwinner
boards.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>