Convert CONFIG_CMD_IMMAP to Kconfig

This converts the following to Kconfig:
   CONFIG_CMD_IMMAP

Also move this command out of the cmd/ directory since it is
PowerPC-specific.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/kmlion1_defconfig b/configs/kmlion1_defconfig
index 287ca8c..56c5a8f 100644
--- a/configs/kmlion1_defconfig
+++ b/configs/kmlion1_defconfig
@@ -1,5 +1,6 @@
 CONFIG_PPC=y
 CONFIG_MPC85xx=y
+CONFIG_CMD_IMMAP=y
 CONFIG_TARGET_KMP204X=y
 CONFIG_FIT=y
 CONFIG_FIT_VERBOSE=y