parse: bootimg: make it work again (MR 2477)

Commit 826bb4f2 (pmb: Properly type Bootimg (MR 2464)) broke the
bootimg_analyze command since some assumptions in whether the bootimg
dictionary had empty keys or non-existent keys were changed without
their usage being updated.

Fix this and apply a trivial cleanup to aportgen.

Signed-off-by: Caleb Connolly <caleb@postmarketos.org>