mkimage: rename a couple of things to be less confusing later.

mkimage: rename a couple of things to be less confusing later.

This renames some things:

- the "strtab" and "strtab_section" in relocate_symbols are changed to "symtab"
  instead, so as to be less confusing when "strtab" is moved to a struct in a
  later patch.

- The places where we pass section_vaddresses to functions are changed to also
  be called section_vaddresses"inside those functions, so I get less confused
  when I put addresses and vaddresses in a struct in a later patch.

Signed-off-by: Peter Jones <pjones@redhat.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>