boot: Consider non-bootable partitions

boot: Consider non-bootable partitions

Any 'bootable' flag in a DOS partition causes boostd to only scan
bootable partitions for that media. This can mean that extlinux.conf
files on the root disk are missed.

Put this logic behind a flag and update the documentation.

For now, the flag is enabled, to preserve the existing behaviour of
bootstd which is to ignore non-bootable partitions so long as there is
at least one bootable partition on the disk.  Future work may provide a
command (or some other mechanism) to control this.

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