live: add karg embed areas in GRUB/ISOLINUX configs

[jlebon]

This provides a buffer into which coreos-installer iso embed-kargs can
write additional kernel arguments for the live ISO. It must be located
right after the karg line in configuration files.

Note this doesn't touch zipl.prm, which is where ISO kernel args are
stored for s390x. Supporting this for s390x would need more
investigation and testing.

[jlebon]

See coreos/coreos-assembler#1637 and coreos/coreos-installer#341 for how this will be used.

[cgwalters]

Just two comments, otherwise LGTM

[bgilbert]

Apologies for the late comment, but would it make sense to use the bootloaders' respective include directives to put these in separate files?

[jlebon]

Apologies for the late comment, but would it make sense to use the bootloaders' respective include directives to put these in separate files?

We need the buffer to be physically located near the kernel cmdline though. The way this works is below the level of the interpreters; coreos-installer just moves the newline character rightwards into the buffer as needed.

I think we could do a more "native" thing for GRUB at least. Something like:

# kargs_embed_area.cfg
set embedded_kargs="" ######...

and then

# grub.cfg
source kargs_embed_area.cfg
linux /images/vmlinuz @@KERNEL-ARGS@@ ignition.firstboot ignition.platform.id=metal $embedded_kargs

For ISOLINUX though, I don't see a way to play the same trick since it doesn't support variables.

[bgilbert]

Ah, right. Okay, seems fine as is. 👍