bitbake & hob: implement functions to assure consistency for configuration files

bitbake & hob: implement functions to assure consistency for configuration files

Added a new command in bitbake to save a variable in a file; added a function
in cooker which is called by this command.

Added new command in bitbake to enable/disable data tracking.

The function saveConfigurationVar from cooker.py saves a variable in the file that
is received by argument. It checks all the operations made on that variable, using the history.
If it's the first time when it does some changes on a variable,it comments the lines where
an operation is made on it, and it sets it in a line to the end of file. If it's not
the first time(it has a comment before), it replaces the line.

Made some changes in hob to save the variables from bblayers.conf and local.conf
using the bitbake command.

[YOCTO #2934]
Signed-off-by: Cristiana Voicu <cristiana.voicu@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>