buildman: Correct operation of -A flag

This was broken when -a was removed and unfortunately there are no tests
for this. Fix it.

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

tools/buildman/control.py

@@ -207,7 +207,7 @@ def DoBuildman(options, args, toolchains=None, make_func=None, boards=None,
         sys.exit(col.Color(col.RED, 'No matching boards found'))
 
     if options.print_prefix:
-        err = ShowToolchainInfo(boards, toolchains)
+        err = ShowToolchainPrefix(boards, toolchains)
         if err:
             sys.exit(col.Color(col.RED, err))
         return 0