Create a new boot/ directory

Quite a lot of the code in common/relates to booting and images. Before
adding more it seems like a good time to move the code into its own
directory.

Most files with 'boot' or 'image' in them are moved, except:

- autoboot.c which relates to U-Boot automatically running a script
- bootstage.c which relates to U-Boot timing

Drop the removal of boot* files from the output directory, since this
interfers with the symlinks created by tools and there does not appear
to be any such file from my brief testing.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Artem Lapkin <email2tema@gmail.com>
Tested-by: Artem Lapkin <email2tema@gmail.com>
diff --git a/Kconfig b/Kconfig
index 931a228..c46f4fc 100644
--- a/Kconfig
+++ b/Kconfig
@@ -466,6 +466,8 @@
 
 source "api/Kconfig"
 
+source "boot/Kconfig"
+
 source "common/Kconfig"
 
 source "cmd/Kconfig"