buildman: Drop the -a option

There is no point in setting the ARCH environment variable since the
U-Boot build system no-longer uses it.

It seems safe to drop this feature since it was only recently added.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/tools/buildman/README b/tools/buildman/README
index 116a0ee..4cf0114 100644
--- a/tools/buildman/README
+++ b/tools/buildman/README
@@ -1072,8 +1072,8 @@
 
 Buildman has various other command-line options. Try --help to see them.
 
-To find out what architecture or toolchain prefix buildman will use for a build,
-see the -a and -A options.
+To find out what toolchain prefix buildman will use for a build, use the -A
+option.
 
 To request that compiler warnings be promoted to errors, use -E. This passes the
 -Werror flag to the compiler. Note that the build can still produce warnings