Convert CONFIG_ENV_IS_IN_FLASH to Kconfig

This converts the following to Kconfig:
   CONFIG_ENV_IS_IN_FLASH

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/ap_sh4a_4a_defconfig b/configs/ap_sh4a_4a_defconfig
index 41c8be9..c08ae4f 100644
--- a/configs/ap_sh4a_4a_defconfig
+++ b/configs/ap_sh4a_4a_defconfig
@@ -1,5 +1,6 @@
 CONFIG_SH=y
 CONFIG_TARGET_AP_SH4A_4A=y
+CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_BOOTDELAY=3
 CONFIG_VERSION_VARIABLE=y
 # CONFIG_CMD_BDI is not set