binman: Support adding files

In some cases it is useful to add a group of files to the image and be
able to access them at run-time. Of course it is possible to generate
the binman config file with a set of blobs each with a filename. But for
convenience, add an entry type which can do this.

Add required support (for adding nodes and string properties) into the
state module.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/README.entries b/tools/binman/README.entries
index 2cf7dc0..3afc560 100644
--- a/tools/binman/README.entries
+++ b/tools/binman/README.entries
@@ -71,6 +71,21 @@
 
 
 
+Entry: files: Entry containing a set of files
+---------------------------------------------
+
+Properties / Entry arguments:
+    - pattern: Filename pattern to match the files to include
+    - compress: Compression algorithm to use:
+        none: No compression
+        lz4: Use lz4 compression (via 'lz4' command-line utility)
+
+This entry reads a number of files and places each in a separate sub-entry
+within this entry. To access these you need to enable device-tree updates
+at run-time so you can obtain the file positions.
+
+
+
 Entry: fill: An entry which is filled to a particular byte value
 ----------------------------------------------------------------