gen_relocate_app: avoid hard-coding section name fragments

This refactors the script to introduce a SectionKind enum that
represents each of the kinds of sections handled by this system (text,
data, rodata or bss), using that to improve the code structure by
reducing the use of strings in favor of indicating the use of values by
their types.

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