Sum of partition sizes in some partitions.conf overflow disk size

[lool]

Hi

Agathe Porte (@gagath) had some issues flashing glymur-crd with images from qcom-deb-images, and worked around by skipping the last partition (dtb_b); abickett noticed that the partitions.conf for glymur-crd (spinor part) has a sum of partition sizes that outgrows the declared --disk size.

We need to:

1. write logic to catch these issues in CI
2. fix all current problematic files, at least glymur-crd/spinor

Adam has shared a WIP patch out of band that I reviewed.

[lool]

Part 2) was started in #98