build: don't assume .symtab section type

Only the elftools SymbolTableSection section type provides an
iter_symbols() method, and compilers are free to emit sections that
include the substring '.symtab' in their name which causes errors if
gen_relocate_app examines only the section name. Instead check whether a
section is a symbol table to skip attempting to inspect sections that
are not actually symbol tables.

Signed-off-by: Peter Marheine <pmarheine@chromium.org>