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/ls1021aqds_qspi_defconfig b/configs/ls1021aqds_qspi_defconfig
index 7993a69..b776ea0 100644
--- a/configs/ls1021aqds_qspi_defconfig
+++ b/configs/ls1021aqds_qspi_defconfig
@@ -8,6 +8,7 @@
 CONFIG_OF_STDOUT_VIA_ALIAS=y
 CONFIG_SYS_EXTRA_OPTIONS="QSPI_BOOT"
 CONFIG_QSPI_BOOT=y
+CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_BOOTDELAY=3
 CONFIG_SILENT_CONSOLE=y
 # CONFIG_CONSOLE_MUX is not set