Add support to chain a hdd boot in the pxelinux parser

[Pierre Neyron]

Hello,

For our project (bare metal deployment service[2] of power8 machines in
our platform[1]), we would like to enable petitboot to chain a local
boot to a disk partition, from a pxelinux directive received from the
network:

1/ petitboot fetches a pxelinux.cfg/XXXXXXXX from the network,
containing for instance:

LABEL local
	COM32 /chain.c32
	APPEND hd0 N

2/ then petitboot boots the matching entry as scanned in the grub.cfg of
the partition => hd0 N -> /dev/sdaN

Questions:
- Is that something that somebody already worked on in the past ? (we
did not find traces of that in the code or documentation)
- Does the required code seems tricky to experienced developers of
petitboot ? (we did not start diving in yet).

Thanks,

[1] https://www.grid5000.fr
[2] https://github.com/madynes/kadeploy
-- 
Pierre NEYRON
Laboratoire d'Informatique de Grenoble
CNRS / Univ-Grenoble-Alpes / Inria