binman: Support positioning an entry by and ELF symbol

In some cases it is useful to position an entry over the top of a symbol
in an ELF file. For example, if the symbol holds a version string then it
allows the string to be accessed from the fdtmap.

Add support for this.

Suggested-by: Pali Rohár <pali@kernel.org>
Suggested-by: Keith Short <keithshort@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/binman.rst b/tools/binman/binman.rst
index 980a1ac..fa8abdc 100644
--- a/tools/binman/binman.rst
+++ b/tools/binman/binman.rst
@@ -823,6 +823,13 @@
     needed. Add this symbol to the properties for the blob so that symbols can
     be read correctly. See binman_syms_ for more information.
 
+offset-from-elf:
+    Sets the offset of an entry based on a symbol value in an another entry.
+    The format is <&phandle>, "sym_name", <offset> where phandle is the entry
+    containing the blob (with associated ELF file providing symbols), <sym_name>
+    is the symbol to lookup (relative to elf-base-sym) and <offset> is an offset
+    to add to that value.
+
 Examples of the above options can be found in the tests. See the
 tools/binman/test directory.