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>

include/bootflow.h

@@ -160,6 +160,7 @@ struct bootflow_img {
  * before using it
  * @BOOTFLOWIF_ALL: Return bootflows with errors as well
  * @BOOTFLOWIF_HUNT: Hunt for new bootdevs using the bootdrv hunters
+ * @BOOTFLOWIF_ONLY_BOOTABLE: Only consider partitions marked 'bootable'
  *
  * Internal flags:
  * @BOOTFLOWIF_SINGLE_DEV: (internal) Just scan one bootdev
@@ -176,6 +177,7 @@ enum bootflow_iter_flags_t {
 	BOOTFLOWIF_SHOW			= 1 << 1,
 	BOOTFLOWIF_ALL			= 1 << 2,
 	BOOTFLOWIF_HUNT			= 1 << 3,
+	BOOTFLOWIF_ONLY_BOOTABLE	= BIT(4),
 
 	/*
 	 * flags used internally by standard boot - do not set these when