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/ls2088ardb_qspi_defconfig b/configs/ls2088ardb_qspi_defconfig
index 76b2f32..3a02233 100644
--- a/configs/ls2088ardb_qspi_defconfig
+++ b/configs/ls2088ardb_qspi_defconfig
@@ -8,6 +8,7 @@
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_OF_STDOUT_VIA_ALIAS=y
 CONFIG_QSPI_BOOT=y
+CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_BOOTDELAY=10
 # CONFIG_CMD_IMLS is not set
 CONFIG_CMD_GREPENV=y