update-rc.d.bbclass: adjust order on tweaking postinst
So far unlike prerm/postrm, update-rc.d has its own postinst method prepended
to pkg_postinst, which may result "System startup links for xxx already exist"
warning in the 1st boot of target image. Some pkg_postinst requires to run on
the target, and thus prepend here makes update-rc.d method executed twice: one
in rootfs creation and the other in the 1st target boot. So adjust the order
to append.
