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/am335x_evm_norboot_defconfig b/configs/am335x_evm_norboot_defconfig
index 0077482..f121e2f 100644
--- a/configs/am335x_evm_norboot_defconfig
+++ b/configs/am335x_evm_norboot_defconfig
@@ -6,6 +6,7 @@
 CONFIG_NOR=y
 CONFIG_DISTRO_DEFAULTS=y
 CONFIG_NOR_BOOT=y
+CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_SYS_CONSOLE_INFO_QUIET=y
 CONFIG_VERSION_VARIABLE=y
 CONFIG_ARCH_MISC_INIT=y