binman: Add an image name into the fdtmap

Since binman supports multiple images it is useful to know which one
created the image that has been read. Then it is possible to look up that
name in the 'master' device tree (containing the description of all
images).

Add a property for this.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/ftest.py b/tools/binman/ftest.py
index 6a40d1f..08a1df0 100644
--- a/tools/binman/ftest.py
+++ b/tools/binman/ftest.py
@@ -2369,7 +2369,7 @@
 '      u-boot            138     4  u-boot            38',
 '      u-boot-dtb        180   10f  u-boot-dtb        80          3c9',
 '    u-boot-dtb          500   %x  u-boot-dtb       400          3c9' % fdt_size,
-'  fdtmap                %x   395  fdtmap           %x' %
+'  fdtmap                %x   3b4  fdtmap           %x' %
         (fdtmap_offset, fdtmap_offset),
 '  image-header          bf8     8  image-header     bf8',
             ]