bitbake.conf: Make TCLIBC and TCMODE lazy assigned

bitbake.conf: Make TCLIBC and TCMODE lazy assigned

This allows two level of overriding (distro level and local.conf/shell
variable). Previous settings blocked shell variables overring
if it was overriden on distro level.

Signed-off-by: Pavel Zhukov <pavel.zhukov@huawei.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit de6c3f9cb2c589aecbf8d9d25fa83cd18bf80891)
Signed-off-by: Steve Sakoman <steve@sakoman.com>