Documentation for dynamically build bootsrap

This blueprint is to describe documentation
that is required for dynamically build ubuntu-based
bootstrap images

Documentation should include:
    1. General description of steps how to build bootstrap image
       from custom repositories
    2. Example how to build bootstrap with custom kernel.
    3. Example how to add custom drivers to the bootstrap image.
    4. Example how to configure PXE parameters (e.g. kernel cmdline).