Recognise build phases (and reuse results)

[vanrein]

Set more refined targets than just ${CURRENT_ROOTFS}-rootfs on which everything hangs. This will involve some ordering of the steps that constitute the rootfs build.

[vanrein]

Also, recognise it when a target has been built at all. It now builds again on any call, even after having succeeded just before!

[vanrein]

These things are caused by a few things:

1. The ${CURRENT_ROOTFS}-rootfs target is made with add_custom_target, so it is always out of date.
2. The use of PRE_BUILD and POST_BUILD relative to that target makes do not introduce their own targets.
3. The actually created items are not always represented in actual files; they are often abstract targets and cannot easily be annotated with an actual target.

The solutions should take the shape of a set of targets that are coupled to files. These need not be actual files for the build, but could be a dedicated touch file.

[vanrein]

Most of the agony is gone, as a result of reuse of the oslibs endeavour in mkhere issue #5.