tools: dumpimage: Remove remaining mentions of the -i option

The -i option of the dumpimage tool has been removed so it should no
longer be documented in the README file. Refer readers to the tool's
help output rather than maintain a copy of the usage in the README.

Finally, adjust the example dumpfile invocation in imagetool.h to use
the -o option instead of the removed -i option.

Fixes: 12b831879a76 ("tools: dumpimage: Simplify arguments")
Signed-off-by: Tyler Hicks <tyhicks@linux.microsoft.com>
Cc: Martyn Welch <martyn.welch@collabora.com>
Acked-by: Martyn Welch <martyn.welch@collabora.com>
diff --git a/README b/README
index cb49aa1..7b73a1c 100644
--- a/README
+++ b/README
@@ -3832,16 +3832,8 @@
 	Load Address: 0x00000000
 	Entry Point:  0x00000000
 
-The "dumpimage" is a tool to disassemble images built by mkimage. Its "-i"
-option performs the converse operation of the mkimage's second form (the "-d"
-option). Given an image built by mkimage, the dumpimage extracts a "data file"
-from the image:
-
-	tools/dumpimage -i image -T type -p position data_file
-	  -i ==> extract from the 'image' a specific 'data_file'
-	  -T ==> set image type to 'type'
-	  -p ==> 'position' (starting at 0) of the 'data_file' inside the 'image'
-
+The "dumpimage" tool can be used to disassemble or list the contents of images
+built by mkimage. See dumpimage's help output (-h) for details.
 
 Installing a Linux Image:
 -------------------------