handle EFI GRUB config paths when setting grubport

Also split the grubport handling into its own snippet and documented it.

Bug: 1108498
Change-Id: Ic9f365fa991594f56ae5c11babb472ceff8c2fe3
fix not-enough-systems logic to correctly abort recipe sets

Under certain very specific circumstances the not-enough-systems logic
would try to move a recipe to Queued after already aborting the recipe
set.

Bug: 1120052
Change-Id: Id64c8a90454013680185e8f214744aa30147d9d9
config option to disable bumping of priority for single-system recipes

Bug: 978904
Change-Id: I4eca275fd1afa4aed4330fb477e72aa9530b4e57
Merge branch 'release-0.17' into develop