doc: Add a note about producing 'md.b' output using hexdump

Comparing a hex dump on the U-Boot command line with the contents of a
file on the host system is fairly easy and convenient to do manually if
it is small. But the format used hexdump by default differs from that
shown by U-Boot. Add a note about how to make them the same.

(For large dumps, writing the data to the network with tftpput, or to a
USB stick with ext4save is easiest.)

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

doc/usage/md.rst

@@ -39,6 +39,16 @@ length
     number of values to dump. Defaults to 40 (0d64). Note that this is not
     the same as the number of bytes, unless .b is used.
 
+Note that the format of 'md.b' can be emulated from linux with::
+
+    # This works but requires using sed to get the extra spaces
+    # <addr> is the address, <f> is the filename
+    xxd -o <addr> -g1 <f> |sed 's/  /    /' >bad
+
+    # This uses a single tool but the offset always starts at 0
+    # <f> is the filename
+    hexdump -v -e '"%08.8_ax: " 16/1 "%02x " "    "' -e '16/1 "%_p" "\n" ' <f>
+
 
 Example
 -------