10.1RC1 armv6 RPI-B stuck in single user mode

[Alban Hertroys]

We have a Raspberry Pi here that was working fine, but because my
colleagues tend to reboot the thing by unplugging the power, the root
file-system got dirty.
To fsck the root file-system, I attempted to boot it into single-user
mode using nextboot -o "-s" -k kernel.

Unfortunately, it never shows the single user mode prompt. The system
gets stuck trying to mount root from ufs:/dev/mmcsd0s2a

And it does that every time I attempt to boot it, because that
nextboot flag never gets cleared!
There doesn't appear to be any method of interrupting the boot loader
on the Pi either.

How do I get out of this pickle?
-- 
If you can't see the forest for the trees,
Cut the trees and you'll see there is no forest.