dpkg: modify the run-postinst script to enable postinst logging

dpkg: modify the run-postinst script to enable postinst logging

Enable postinst logging by checking the configuration in ${sysconfdir}
/default/postinst.

In this way, the postinst logging is enabled if 'debug-tweaks' is
in IMAGE_FEATURES, and at the same time, we avoid unnecessary rebuilt
if IMAGE_FEATURES is changed.

[YOCTO #4262]

(From OE-Core rev: 881880992ac0edc5f928f7e3d2a8f3f993284df6)

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>