bootstd: Add some default filesystems and commands

We need to support a basic set of filesystems for booting to work in most
cases. Add these in via a new option, letting the board disable them
individually (for space reasons) if desired.

This enables the filesystem commands as well as the actual functionality,
even though bootstd is quite happy to use ext4 without the ext4 command.
Further work would be needed to disintangle this and reduce code size.

Add several other options as well, providing sensible defaults.

We cannot enable this by default, since it expands the size of many
boards quite a lot.

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