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/README b/README
index 0f52888..0a71933 100644
--- a/README
+++ b/README
@@ -144,6 +144,7 @@
   /xtensa		Files generic to Xtensa architecture
 /api			Machine/arch-independent API for external apps
 /board			Board-dependent files
+/boot			Support for images and booting
 /cmd			U-Boot commands functions
 /common			Misc architecture-independent functions
 /configs		Board default configuration files