yaml srclib support

Basic support for yaml srclibs.

outline of these changes:

• srclibs can now be specified as srclibs/___.yml files in yaml format
• support for Prepare as list. (list-items get concatenated with &&)
• support for Subdir as list. (old , separated lists still work in both txt and yml format)
• support for txt format srclibs remains untouched
• added tests for txt and yaml srclib parsing
• support for copying yml srclibs into a buildserver VM

closes #609 (closed)