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/ls1046ardb_qspi_defconfig b/configs/ls1046ardb_qspi_defconfig
index 0a8f1a0..f4ff5d6 100644
--- a/configs/ls1046ardb_qspi_defconfig
+++ b/configs/ls1046ardb_qspi_defconfig
@@ -6,6 +6,7 @@
 CONFIG_FIT_VERBOSE=y
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_QSPI_BOOT=y
+CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_BOOTDELAY=10
 CONFIG_HUSH_PARSER=y
 # CONFIG_CMD_IMLS is not set