Generate boot/grub/loopback.cfg for Grub2 loopback.cfg support

Must be applied with the "Parse and use isoloop= kernel command line variable" patch to alpine/mkinitfs/initramfs.in to work correctly.
Implements alpine/aports#5384
Status Job ID Name Coverage
  Lint
passed #143542
docker-alpine x86_64
lint

00:00:25

 
  Build
passed #143547
docker-alpine ci-build aarch64
build-aarch64

00:00:24

passed #143548
docker-alpine ci-build armv7
build-armv7

00:00:30

passed #143546
docker-alpine ci-build ppc64le
build-ppc64le

00:00:37

passed #143545
docker-alpine ci-build s390x
build-s390x

00:00:20

passed #143544
docker-alpine ci-build x86
build-x86

00:00:19

passed #143543
docker-alpine ci-build x86_64
build-x86_64

00:00:21