postfix: fixed installing to the filesystem image

Fix for issues #491 and #492. Parts of the postinst script are moved into init script to ensure that they are executed on a target device. The remaining parts of the postinst script are OK to be executed on build system if postfix is chosen to be built as a part of the filesystem image.

Signed-off-by: Denis Shulyaka <Shulyaka@gmail.com>