scripts/mkimg.base.sh: add openntpd to base

fixes #10080
8 jobs for !23651 with mkimg-base-openntpd in 1 minute and 10 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Lint
passed #448138
docker-alpine x86_64
lint

00:00:23

 
  Build
passed #448143
docker-alpine ci-build aarch64
build-aarch64

00:00:25

passed #448145
docker-alpine ci-build armhf
build-armhf

00:00:21

passed #448144
docker-alpine ci-build armv7
build-armv7

00:00:21

passed #448142
docker-alpine ci-build ppc64le
build-ppc64le

00:00:32

passed #448141
docker-alpine ci-build s390x
build-s390x

00:00:20

passed #448140
docker-alpine ci-build x86
build-x86

00:00:12

passed #448139
docker-alpine ci-build x86_64
build-x86_64

00:00:15