bitbake: main: add 2 environment variables

bitbake: main: add 2 environment variables

Bitbake uses set of environment variables to set command line
options, e.g. seeting BBTOKEN variable has the same effect
as using --token command line option.

Added new environment variables BBPRECONF and BBPOSTCONF that
are equivalents of --read and --postread command line options.
They can be used by high level scripts to append or prepend
configuration files to conf/local.conf

[YOCTO #9235]

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