Add a function to init a bootflow, to reduce code duplication.

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

bootdev_list() uses uclass_*_device_err() to iterate devices.
However, the only value _err adds is returning an error when the device
pointer is null, and that's checked anyway.

Also there is some intent to report errors, and that's what
uclass_*_device_check() is for, use it.

Signed-off-by: Michal Suchanek <msuchanek@suse.de>
Reviewed-by: Simon Glass <sjg@chromium.org>

With global bootmeths we want to scan without a bootdev. Update the logic
to allow this.

Change the bootflow command to show the bootdev only when valid.

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

Avoid using 'count' to mean either a count or an error, since this is
confusing. In fact, the called function never return 0, since that is an
error.

Use 'ret' instead.

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

Add support for bootflows, including maintaining a list of them and
iterating to find them.

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

A 'bootdev' is a device which can be used to boot an operating system.
It is a child of the media device (e.g. MMC) which handles reading files
from that device, such as a bootflow file.

Add a uclass for bootdev and the various helpers needed to make it
work. Also add a binding file, empty for now.

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