build_image: don't call append2simg for FEC

append2simg can be slow, so generate error correction data before verity
data is appended, and append all metadata to the image at the same time.

Needs matching changes from
  I09349368925e5e2f2a7961a712a1e933eb0b9ec8

Bug: 26251929
Change-Id: Iea8cee0aee353b4d029d9581e8fbd68ec7b7dd4b