package_deb: skip pre/postrm scripts on upgrade, write only one shebang

package_deb: skip pre/postrm scripts on upgrade, write only one shebang

Trying to upgrade busybox removing symlinks but update-alternatives
need these links (sed, cut, tail, etc) in order to work.

Adding test to avoid this scripts on upgrade fix the problem, same
solution are found in package_rpm class.

[YOCTO #6768]

Signed-off-by: Andreas Oberritter <obi@opendreambox.org>
Signed-off-by: Aníbal Limón <anibal.limon@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>