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/P1021RDB-PC_SPIFLASH_defconfig b/configs/P1021RDB-PC_SPIFLASH_defconfig
index bdb0103..306d610 100644
--- a/configs/P1021RDB-PC_SPIFLASH_defconfig
+++ b/configs/P1021RDB-PC_SPIFLASH_defconfig
@@ -12,6 +12,7 @@
 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