bootstd: Add a new bootmeth method to set the bootflow

Normally the bootmeth driver reads the bootflow from the bootdev, since
it knows the correct way to do it.

However it is easier for some bootdevs to handle this themselves. For
example, reading from SPI flash is quite different from other devices.

Add a way for the bootdev to pass a bootflow to the bootmeth, so that
this can be supported.

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