Convert CONFIG_ENV_IS_IN_SPI_FLASH to Kconfig

This converts the following to Kconfig:
   CONFIG_ENV_IS_IN_SPI_FLASH

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/ge_b850v3_defconfig b/configs/ge_b850v3_defconfig
index 967ddba..574e916 100644
--- a/configs/ge_b850v3_defconfig
+++ b/configs/ge_b850v3_defconfig
@@ -2,6 +2,7 @@
 CONFIG_ARCH_MX6=y
 CONFIG_TARGET_GE_B850V3=y
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
+CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_BOOTDELAY=1
 CONFIG_SYS_CONSOLE_IS_IN_ENV=y
 CONFIG_SYS_CONSOLE_OVERWRITE_ROUTINE=y