calculation of disk partition sizes is messy.

[tolland]

So currently, to build the recovery partition, we need to know the following:

1. what is the source of the recovery partition filesystem (i.e. is it a copy of the rootfs, or some slim/minimal filesystem?)
2. how much free space is available in the recovery partition
3. how big is the zipped rootfs file? (as it needs to fit in the free space in the recovery filesystem)
4. how much free space padding is required for the recoveryfs to boot and do stuff (like logging, journal, caches etc)

These values need to be calculated, and the recovery partition and filesystem need to be sized accordingly.

These values can vary wildly depending on how big the zipped rootfs ends up being (depending on it being lite, desktop or full Pi OS etc) and also how much spare space is available on the recovery filesystem.

Currently the code to do this is nasty, and made more difficult by using bash to do the arithmetic