bootstd: Add a command to read all files for a bootflow

Some bootflows (such as EFI and ChromiumOS) delay reading the kernel until
it is needed to boot. This saves time when scanning and avoids needing to
allocate memory for something that may never be used.

To permit reading of these files, add a new 'bootflow read' command.

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