bootstd: Add a simple command to list images

Add a new 'bootstd images' command, which lists the images which have
been loaded.

Update some existing tests to use it. Provide some documentation about
images in general and this command in particular.

Use a more realistic kernel command-line to make the test easier to
follow.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/cmd/Makefile b/cmd/Makefile
index d1f369d..c0b0ff8 100644
--- a/cmd/Makefile
+++ b/cmd/Makefile
@@ -23,6 +23,7 @@
 obj-$(CONFIG_CMD_BOOTDEV) += bootdev.o
 obj-$(CONFIG_CMD_BOOTFLOW) += bootflow.o
 obj-$(CONFIG_CMD_BOOTMETH) += bootmeth.o
+obj-$(CONFIG_CMD_BOOTSTD) += bootstd.o
 obj-$(CONFIG_CMD_SOURCE) += source.o
 obj-$(CONFIG_CMD_BCB) += bcb.o
 obj-$(CONFIG_CMD_BDI) += bdinfo.o