dpkg, opkg, rpm-postinst: fix overwriting the run-postinstall script

dpkg, opkg, rpm-postinst: fix overwriting the run-postinstall script

If multiple package managers are installed in the image, they will
overwrite each other's run-postinsts script, resulting in postinstalls
not beeing run at all at first boot.

What this patch does:
 * checks whether opkg/dpks/rpm is actually used to install
   the packages and, only after, creates the run-postinsts script;
 * brings dpkg recipe in sync with opkg: moves the script creation from
   do_install to postinstall;
 * move creation of run-postinsts script (rpm-postinsts recipe) to the
   postinstall scriptlet in order to better control the creation of the
   script according to the package manager used;

[YOCTO #4231]
[YOCTO #4179]

(From OE-Core rev: d7fd56df0a4954954d6d0764ae06beb869e6b99a)

Signed-off-by: Laurentiu Palcu <laurentiu.palcu@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>