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>
diff --git a/tools/binman/README.entries b/tools/binman/README.entries
index 357946d..702fc9f 100644
--- a/tools/binman/README.entries
+++ b/tools/binman/README.entries
@@ -206,6 +206,8 @@
 
 A typical filename is 'me.bin'.
 
+The position of this entry is generally set by the intel-descriptor entry.
+
 See README.x86 for information about x86 binary blobs.