binman: Add a function to read ELF symbols

In some cases we need to read symbols from U-Boot. At present we have a
a few cases which does this via 'nm' and 'grep'.

It is better to use objdump since that tells us the size of the symbols
and also whether it is weak or not.

Add a new module which reads ELF information from files. Update existing
uses of 'nm' to use this module.

Signed-off-by: Simon Glass <sjg@chromium.org>

tools/binman/test/u_boot_ucode_ptr.c

@@ -4,7 +4,7 @@
  * SPDX-License-Identifier:     GPL-2.0+
  *
  * Simple program to create a _dt_ucode_base_size symbol which can be read
- * by 'nm'. This is used by binman tests.
+ * by binutils. This is used by binman tests.
  */
 
 static unsigned long _dt_ucode_base_size[2]