bootm: Tidy up use of autostart env var

This has different semantics in different places. Go with the bootm method
and put it in a common function so that the behaviour is consistent in
U-Boot. Update the docs.

To be clear, this changes the way that 'bootelf' and standalone boot
work. Before, if autostart was set to "fred" or "YES", for example, they
would consider that a "yes". This may change behaviour for some boards,
but the only in-tree boards which mention autostart use "no" to disable
it, which will still work.

Signed-off-by: Simon Glass <sjg@chromium.org>
Suggested-by: Wolfgang Denk <wd@denx.de>

doc/usage/environment.rst

@@ -179,8 +179,9 @@ autostart
     be automatically started (by internally calling
     "bootm")
 
-    If set to "no", a standalone image passed to the
-    "bootm" command will be copied to the load address
+    If unset, or set to "1"/"yes"/"true" (case insensitive, just the first
+    character is enough), a standalone image
+    passed to the "bootm" command will be copied to the load address
     (and eventually uncompressed), but NOT be started.
     This can be used to load and uncompress arbitrary
     data.