bitbake: hob: fix set_extra_setting function

bitbake: hob: fix set_extra_setting function

The function is used to save additional variables in the configuration file
when the user adds a new (key, value) pair from the Settings->Others. There
was a problem though when the function was trying to retrieve an older
instance of EXTRA_SETTINGS from the configuration file. Sometimes its value
was returned as a dictionary and sometimes a string, which caused a crash when
calling ast.literal_eval(). The reason of the problem must be a change in
bitbake's parsing system. The changes will fix this issues.

While analysing this problem I discovered that the variables were not saved
properly in the configuration file after consecutive changes to Settings->Others
because of the way saveConfigurationVar() from cooker.py works. This patch
will also solve this issue.

[YOCTO #5989]

(Bitbake rev: bdbcd8866104c315fc9da631407d4280433dbfde)

Signed-off-by: Marius Avram <marius.avram@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>