bootstd: Use the bootargs env var for changing the cmdline

The "bootargs" environment variable is used to set the command-line
arguments to pass to the OS. Use this same mechanism with bootstd as well.
When the variable is updated, it is written to the current bootflow. When
the current bootflow is updated, the environment variable is updated too.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

include/env_callback.h

@@ -60,8 +60,10 @@
 #define NET6_CALLBACKS
 #endif
 
-#ifdef CONFIG_BOOTSTD
-#define BOOTSTD_CALLBACK	"bootmeths:bootmeths,"
+#ifdef CONFIG_BOOTSTD_FULL
+#define BOOTSTD_CALLBACK \
+	"bootmeths:bootmeths," \
+	"bootargs:bootargs,"
 #else
 #define BOOTSTD_CALLBACK
 #endif