bootstd: Allow scanning a single bootdev label

We want to support scanning a single label, like 'mmc' or 'usb0'. Add
this feature by plumbing the label through to the iterator, setting a
flag to indicate that only siblings of the initial device should be used.

This means that scanning a bootdev by its name is not supported anymore.
That feature doesn't seem very useful in practice, so it is no great loss.

Add a test for bootdev_find_by_any() while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/test/boot/bootdev.c b/test/boot/bootdev.c
index 679ffc4..1724e34 100644
--- a/test/boot/bootdev.c
+++ b/test/boot/bootdev.c
@@ -626,7 +626,7 @@
 	struct udevice *dev;
 	int ret;
 
-	sandbox_set_eth_enable(false);
+	test_set_eth_enable(false);
 	test_set_skip_delays(true);
 
 	/* get access to the used hunters */