part: Add a function to find the first bootable partition

If a disk has a bootable partition we are expected to use it to locate the
boot files. Add a function to find it.

To test this, update mmc1 to have two paritions, fixing up other tests
accordingly.

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

include/part.h

@@ -303,6 +303,14 @@ part_get_info_by_dev_and_name_or_num(const char *dev_iface,
 }
 #endif
 
+/**
+ * part_get_bootable() - Find the first bootable partition
+ *
+ * @desc: Block-device descriptor
+ * @return first bootable partition, or 0 if there is none
+ */
+int part_get_bootable(struct blk_desc *desc);
+
 struct udevice;
 /**
  * part_create_block_devices - Create block devices for disk partitions