bootstd: Add the bootmeth uclass and helpers

A bootmeth is a method of locating an operating system. For now, just
add the uclass itself. Drivers for particular bootmeths are added later.

If no bootmeths devices are included in the devicetree, create them
automatically. This avoids the need for boilerplate in the devicetree
files.

Signed-off-by: Simon Glass <sjg@chromium.org>

include/env_callback.h

@@ -57,6 +57,12 @@
 #define NET_CALLBACKS
 #endif
 
+#ifdef CONFIG_BOOTSTD
+#define BOOTSTD_CALLBACK	"bootmeths:bootmeths,"
+#else
+#define BOOTSTD_CALLBACK
+#endif
+
 /*
  * This list of callback bindings is static, but may be overridden by defining
  * a new association in the ".callbacks" environment variable.
@@ -65,6 +71,7 @@
 	ENV_DOT_ESCAPE ENV_FLAGS_VAR ":flags," \
 	"baudrate:baudrate," \
 	NET_CALLBACKS \
+	BOOTSTD_CALLBACK \
 	"loadaddr:loadaddr," \
 	SILENT_CALLBACK \
 	SPLASHIMAGE_CALLBACK \