cooker: preserve pre and post configs

cooker: preserve pre and post configs

Additional config files passed to bitbake server with --read and
--postread options are rewritten by client bitbake even if
it doesn't use those options.

This is a show stopper for toaster as toaster command line
builds are based on the assumption that server is aware of
toster configs, provided by --postread option.

This behaviour is fixed by preserving values of --read and
--postread options when bitbake server starts and restoring
them if client bitbake doesn't explicitly specify them.

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>