binman: Support an entry that holds text

binman: Support an entry that holds text

It is useful to able to write an identifying string to the image within an
entry. Add a 'text' entry type to handle this. The actual text is
typically passed to binman on the command line. The text is not itself
nul-terminated but this can be achieved if required by setting the size of
the entry to something larger than the text.

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