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/test/boot/bootdev.c b/test/boot/bootdev.c
index 5f07430..d549991 100644
--- a/test/boot/bootdev.c
+++ b/test/boot/bootdev.c
@@ -509,6 +509,7 @@
 	iter.part = 0;
 	ut_assertok(uclass_get_device_by_name(UCLASS_BLK, "mmc1.blk", &blk));
 	iter.dev = blk;
+	iter.flags = BOOTFLOWIF_ONLY_BOOTABLE;
 	ut_assertok(device_find_next_child(&iter.dev));
 	uclass_first_device(UCLASS_BOOTMETH, &bflow.method);