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/test/files/1.dat b/tools/binman/test/files/1.dat
new file mode 100644
index 0000000..a952470
--- /dev/null
+++ b/tools/binman/test/files/1.dat
@@ -0,0 +1 @@
+sorry I'm late