bitbake/hob: removing extra parameters from conf files using hob

bitbake/hob: removing extra parameters from conf files using hob

In Hob settings, there is a tab to add/remove extra settings. This
patch implements a way to "remove" variables from conf files, through
bitbake. But, to keep the history assigment of the variables synchronized,
instead of removing, it replaces the lines with blank lines.

[YOCTO #5284]
Signed-off-by: Cristiana Voicu <cristiana.voicu@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>