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
x86_64 docker-alpine
lint

00:00:25

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

00:00:24

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

00:00:30

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

00:00:37

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

00:00:20

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

00:00:19

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

00:00:21