scripts: build: elf_parser: retrieve section directly

Retrieve the section holding the symbol data directly from the symbol
information, instead of trying to find a section that contains data at
the correct address. The later approach does not work for relocatable
files, which contain multiple sections all containing data in
overlapping temporary memory addresses (usually starting at 0).

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>