mtd: afs: factor footer parsing into the v1 part parsing

This simplifies the code by factoring in the image footer
parsing into the single function parsing the AFSv1 partitions.

Cc: Ryan Harkin <ryan.harkin@linaro.org>
Acked-by: Liviu Dudau <liviu.dudau@arm.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Richard Weinberger <richard@nod.at>