initram: kill .apk-new files after installing newroot

and add a boot option keep_apk_new to keep them. Might be useful during
upgrades.

initramfs-init

@@ -182,6 +182,11 @@ done
 apk add --root /newroot --initdb --quiet --progress $pkgs
 eend $?
 
+# kill .apk-new files
+if [ -z "$KOPT_keep_apk_new" ]; then
+	find /newroot/etc -name '*.apk-new' | xargs rm
+fi
+
 # copy alpine release info
 cp $ALPINE_MNT/.alpine-release $NEWROOT/
 ln -sf /.alpine-release $NEWROOT/etc/alpine-release