Simplify and correct the bm partition sizes.

The sfdisk code that landed uses -D, which ends up subtracting space
from the first partition - this plays havoc with precisely sized disk
images due to a bug in the automatic adjustment code. Instead we
directly specify the spare space to be allowed.

Additionally, sfdisk can be used much more simply, avoiding any room
for us to make arithmetic errors, and reducing the opportunity for
race conditions with udev so this change does that.

Change-Id: I7f95e867d8775f49698cbbf4b1be40e69db9d538