patman: Replace the tool with a stub for patch-manager

patman is now maintained as a standalone 'patch-manager' package, rather
than in the U-Boot tree. Replace the command with a small stub which
tells people how to install it.

buildman still uses the shared modules commit and patchstream (and their
dependencies), so leave those in place; the patches that follow remove
the patch-management code itself.

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