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/P1020RDB-PC_36BIT_SPIFLASH_defconfig b/configs/P1020RDB-PC_36BIT_SPIFLASH_defconfig
index 324c6a0..13ecc32 100644
--- a/configs/P1020RDB-PC_36BIT_SPIFLASH_defconfig
+++ b/configs/P1020RDB-PC_36BIT_SPIFLASH_defconfig
@@ -13,6 +13,7 @@
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_OF_STDOUT_VIA_ALIAS=y
 CONFIG_SYS_EXTRA_OPTIONS="SPIFLASH"
+CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_BOOTDELAY=10
 CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_SPL=y