Loopback cfg support.

Development details
--------------------

* This patch has been based originally on: binary_grub2 . It has been
improved thanks to some binary_syslinux bits.

* This patch ensures that binary_loopback_cfg needs is run
before binary_syslinux is run.
The reason is that it reuses some code from binary_syslinux to avoid
problems when binary_syslinux renames the kernel filenames.

* This patch already supports CPU detection

* I haven't tested all the possible scenarios for the script (with the
latest version). When amd64 and 486 Gnu/Linux flavours is used and the
bootloader is syslinux it works ok.

* I have not implemented a disable switch for not generating it.
Usually you always want loopback.cfg to be there.

* Compared to binary_grub2 script I have removed the installation
entries because I did not see any of them in binary_syslinux.

How to test
-----------

These are some steps to easily test if Looback cfg support is working ok.

0) We assume you have generated an iso
1) Make sure you have a partition that Grub understands. Plain ext4 or
vfat should do it.
2) Create directory: /boot/boot-isos/
3) Put the iso file into that directory making sure it has an ISO or iso
extension.
4) Setup your computer to boot from cdrom and use: Super Grub2 Disk
2.00s2 (Hybrid version recommended)
5) Choose Boot manually...
6) Choose Bootable ISOs (in /boot- ... )
7) Choose (the detected) GRUB Loopback Config
(hdN,msodsN)/boot/boot-ios/name-of-the.iso
8) You will be presented your loopback.cfg. Choose anyone of the entries
(unless it does not match your cpu architecture of course).
9) You should boot into your Debian Live without problems (thanks to
findiso boot parametre).

If you ever wanted to test from your grub2 installation instead from
Super Grub2 Disk check: http://www.supergrubdisk.org/wiki/Loopback.cfg
for an example.