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/Kconfig b/arch/powerpc/Kconfig
index e9002a7..8094416 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -28,6 +28,7 @@
 	bool "MPC86xx"
 	select SYS_FSL_DDR
 	select SYS_FSL_DDR_BE
+	imply ENV_IS_IN_FLASH
 
 config 8xx
 	bool "MPC8xx"