bootstd: Mention FS state in bootmeth_read_bootflow()

Add a comment to help implementers deal with the need for calling
fs_set_blk_dev_with_part()

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

Series-to: u-boot
Cover-letter:
bootstd: Useability improvements
This series collects together some bootstd improvements:

- Improve iteration when there are a lot of devices
- Add a test image for Ubuntu (to compliment Fedora)
- Improve the naming of USB devices and bootdevs
- Add a new command to set the bootdev order
- Add a little more debugging
- Use an abuf when dealing with allocate files
- A few other minor things in bootstd

Most of these issues were found when adding the new test image and more
USB devices to sandbox.

include/bootmeth.h

@@ -213,6 +213,9 @@ int bootmeth_check(struct udevice *dev, struct bootflow_iter *iter);
 /**
  * bootmeth_read_bootflow() - set up a bootflow for a device
  *
+ * On entry fs_set_blk_dev_with_part() has been called so it should be possible
+ * to read the file without calling that again.
+ *
  * @dev:	Bootmethod device to check
  * @bflow:	On entry, provides dev, hwpart, part and method.
  *	Returns updated bootflow if found