rockchip: make_fit_atf.py: Eliminate pyelftools dependency

make_fit_aft.py depends on the non-standard library pyelftools to pull
out PT_LOAD segments from ELF files. However, this is as easy to do
manually, without imposing the extra dependency on users.

Structures in the ELF file are unpacked into variables named to exactly
match the ELF spec to ensure the destructuring code is reasonably
self-documenting.

Signed-off-by: Chris Webb <chris@arachsys.com>
Reviewed-by: Andy Yan <andy.yan@rock-chips.com>
Reviewed-by: Kever Yang <Kever.yang@rock-chips.com>