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>

bootstd: Add setup for the bootflow tests

We need to create a disk image with a partition table and a DOS-format
filesystem containing a few files. Provide a fallback binary for CI since
it does not seem able to detect the loopback partitions.

Add this to a dm_init test so that it happens when needed.

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

bootstd: Add setup for the bootflow tests

We need to create a disk image with a partition table and a DOS-format
filesystem containing a few files. Provide a fallback binary for CI since
it does not seem able to detect the loopback partitions.

Add this to a dm_init test so that it happens when needed.

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