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/sh7752evb_defconfig b/configs/sh7752evb_defconfig
index 4e19243..1271ea0 100644
--- a/configs/sh7752evb_defconfig
+++ b/configs/sh7752evb_defconfig
@@ -1,6 +1,7 @@
 CONFIG_SH=y
 CONFIG_SH_32BIT=y
 CONFIG_TARGET_SH7752EVB=y
+CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_BOOTDELAY=3
 CONFIG_VERSION_VARIABLE=y
 # CONFIG_CMD_BDI is not set