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/powerpc/cpu/mpc86xx/Kconfig b/arch/powerpc/cpu/mpc86xx/Kconfig
index 2cc180d..fe56efd 100644
--- a/arch/powerpc/cpu/mpc86xx/Kconfig
+++ b/arch/powerpc/cpu/mpc86xx/Kconfig
@@ -40,6 +40,7 @@
 	select FSL_LAW
 	select SYS_FSL_HAS_DDR1
 	select SYS_FSL_HAS_DDR2
+	imply ENV_IS_IN_FLASH
 
 config FSL_LAW
 	bool