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>
diff --git a/cmd/bootflow.c b/cmd/bootflow.c
index 6d0be32..5349abe 100644
--- a/cmd/bootflow.c
+++ b/cmd/bootflow.c
@@ -173,7 +173,7 @@
 
 	std->cur_bootflow = NULL;
 
-	flags = 0;
+	flags = BOOTFLOWIF_ONLY_BOOTABLE;
 	if (list)
 		flags |= BOOTFLOWIF_SHOW;
 	if (all)