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/socfpga_is1_defconfig b/configs/socfpga_is1_defconfig
index 058791e..371c630 100644
--- a/configs/socfpga_is1_defconfig
+++ b/configs/socfpga_is1_defconfig
@@ -5,6 +5,7 @@
 CONFIG_SPL_STACK_R_ADDR=0x00800000
 CONFIG_DEFAULT_DEVICE_TREE="socfpga_cyclone5_is1"
 CONFIG_FIT=y
+CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_SYS_CONSOLE_IS_IN_ENV=y
 CONFIG_SYS_CONSOLE_OVERWRITE_ROUTINE=y
 CONFIG_SYS_CONSOLE_ENV_OVERWRITE=y