buildbot config: simplified a lot and cleaned

buildbot config: simplified a lot and cleaned

- it is configurable by few variables:

  - machines_glibc/machines_uclibc lists MACHINE values for each libc build
    (so atngw100 can be used just for uclibc)
  - targets_glibc/targets_uclibc lists targets (can be image or any other
    argument for bitbake)

- ANGSTROMLIBC and MACHINE are passed only by environment now so less
  parameters are given to bbot_build_oe_target (RunOEImage before
  renaming).

- initialization of factories is done in one function
  bbot_initialize_factory which gets few arguments:
  - factory
  - clean $TMPDIR (default False)
  - URL to git repository
  - git branch