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>