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/arch/sh/Kconfig b/arch/sh/Kconfig
index 6ac22af..1d40fe6 100644
--- a/arch/sh/Kconfig
+++ b/arch/sh/Kconfig
@@ -3,13 +3,16 @@
 
 config CPU_SH2
 	bool
+	imply ENV_IS_IN_FLASH
 
 config CPU_SH2A
 	bool
 	select CPU_SH2
+	imply ENV_IS_IN_FLASH
 
 config CPU_SH3
 	bool
+	imply ENV_IS_IN_FLASH
 
 config CPU_SH4
 	bool