binman: Don't assume there is an ME region

binman: Don't assume there is an ME region

At present having a descriptor means that there is an ME (Intel
Management Engine) entry as well. The descriptor provides the ME location
and assumes that it is present.

For some SoCs this is not true. Before providing the location of a
potentially non-existent entry, check if it is present.

Update the comment in the ME entry also.

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