bitbake: toaster-requirements.txt: document requirements for the python environment

bitbake: toaster-requirements.txt: document requirements for the python environment

Since Toaster adds a number of specific requirements managed
by pip, we document these requirements in order to be able
to run bitbake and toaster under virtualenv. The target here
is to break the dependency on distro packages for specific
python libraries.

In order to start bitbake/Toaster in a distro-independent manner,
we use virtualenv and pip. We add venv to the .gitignore list
to make room for the virtualenv deployment in this directory.

Use this command sequence to setup the virtualenv:

$ virtualenv venv
$ . venv/bin/activate
$ cat toaster-requirements.txt | xargs pip install

bitbake and Toaster will then run normally under virtualenv.

We also fix bugs related to toaster starting in managed mode
through this script:
  * toaster will not use bldcontrol application in interactive mode
  * a proper message will be displayed if toaster detects a managed
start request in a build environment

(Bitbake rev: b5789c7ea7e6d1a4b67ec60fc0e4233ec543ba3a)

Signed-off-by: Alexandru DAMIAN <alexandru.damian@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>