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>

doc/develop/bootstd/overview.rst

@@ -235,8 +235,9 @@ means that `default_get_bootflow()` is used. This simply obtains the
 block device and calls a bootdev helper function to do the rest. The
 implementation of `bootdev_find_in_blk()` checks the partition table, and
 attempts to read a file from a filesystem on the partition number given by the
-`@iter->part` parameter. If there are any bootable partitions in the table,
-then only bootable partitions are considered.
+`@iter->part` parameter. If there are any bootable partitions in the table and
+the BOOTFLOWIF_ONLY_BOOTABLE flag is set in `@iter->flags`, then only bootable
+partitions are considered.
 
 Each bootdev has a priority, which indicates the order in which it is used,
 if `boot_targets` is not used. Faster bootdevs are used first, since they are