build_image: add padding to match partition size

For some partition sizes, we currently build an image that's 1-2
blocks smaller than the actual partition, which causes fs_mgr to
not find metadata. This change adds padding to FEC metadata that
correctly positions the metadata header at the end.

Bug: 28865197
Change-Id: Ie0e044715a9c5ae8ba395e7d2ff9fbd7cffc0b4c