env: x86: braswell: Set ENV_IS_IN_SPI_FLASH as default

Imply does not work for a Kconfig choice. Update ENV_IS_IN_SPI_FLASH
to be the default one for Intel Braswell.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>