dropbear: Fix incorrect package override for postrm

This bug has been around for a long time (2011) but fix it to do
what was intended originally. The postrm is changed by classes but
those should append to existing entries.

(From OE-Core rev: 7762b07bf3b0c77b91ad91963ab39d3c73770ebf)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-core/dropbear/dropbear.inc

@@ -114,7 +114,7 @@ ALTERNATIVE:${PN} = "${@bb.utils.filter('BINCOMMANDS', 'scp ssh', d)}"
 
 ALTERNATIVE_TARGET = "${sbindir}/dropbearmulti"
 
-pkg_postrm:append_${PN} () {
+pkg_postrm:${PN} () {
   if [ -f "${sysconfdir}/dropbear/dropbear_rsa_host_key" ]; then
         rm ${sysconfdir}/dropbear/dropbear_rsa_host_key
   fi