bitbake: cooker: Add option of specifying indvidual tasks for targets

bitbake: cooker: Add option of specifying indvidual tasks for targets

Currently its near enough impossible to tell bitbake to run target X,
task Y and target A, task B. We could hack various parts of the API
around but it would mean incompatibilities.

An alternative is to accept the syntax "<target>:do_<task>" as a target
name. The default task would be used where the task is unspecified.

This has the advantage that its neat/clean code and works from all
current APIs including the commandline.

(Bitbake rev: 55f6bee3114e582333a1784caeddb197b9163d02)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>