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/stv0991_defconfig b/configs/stv0991_defconfig
index c91b8b7..6940696 100644
--- a/configs/stv0991_defconfig
+++ b/configs/stv0991_defconfig
@@ -3,6 +3,7 @@
 CONFIG_SYS_MALLOC_F_LEN=0x2000
 CONFIG_DEFAULT_DEVICE_TREE="stv0991"
 CONFIG_SYS_EXTRA_OPTIONS="STV0991"
+CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_BOOTDELAY=3
 # CONFIG_DISPLAY_CPUINFO is not set
 # CONFIG_DISPLAY_BOARDINFO is not set