lib: Support printing an error string

It is often useful to show an error code to give the user a clue as to
what went wrong. When error strings are compiled into U-Boot it is
possible to show a message as well.

But at present it is not very convenient, since code must check if the
error strings are present, then obtain the error string and use it in
a printf() string.

Add a %dE option which shows an error code along with an error string,
if available. This makes it easy to show one or both.

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