Mark DISTRO_DEFAULTS as deprecated

Standard boot has been in place for a while now. Quite a few problems
have been found and fixed. It seems like a good time to mark the
script-based approach as deprecated and encourage people to use standard
boot.

Update the DISTRO_DEFAULTS Kconfig to encourage people to move to
standard boot, which is able to boot Linux distributions automatically.

Add a short migration guide to make this easier.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/boot/Kconfig b/boot/Kconfig
index 4ebcf31..7ef44a2 100644
--- a/boot/Kconfig
+++ b/boot/Kconfig
@@ -784,7 +784,7 @@
 endmenu		# Boot images
 
 config DISTRO_DEFAULTS
-	bool "Select defaults suitable for booting general purpose Linux distributions"
+	bool "(deprecated) Script-based booting of Linux distributions"
 	select BOOT_DEFAULTS
 	select AUTO_COMPLETE
 	select CMDLINE_EDITING
@@ -792,6 +792,10 @@
 	select HUSH_PARSER
 	select SYS_LONGHELP
 	help
+	  Note: These scripts have been replaced by Standard Boot. Do not use
+	  them on new boards. See 'Migrating from distro_boot' at
+	  doc/develop/bootstd.rst
+
 	  Select this to enable various options and commands which are suitable
 	  for building u-boot for booting general purpose Linux distributions.