Add support for BIOS/extlinux setups

[bstaletic]

I've only implemented the BIOS/legacy boot + extlinux combination, because that's what I am using.

The current krenel_re matches lines like these:

    KERENEL /boot/vmlinuz
    LINUX /boot/vmlinuz.old

A few things to discuss there:

1. KERNEL tells extlinux to guess the format of the kernel image, which is why LINUX is preferred for linux kernels. Do we want to support KERNEL?
2. Absolute paths in extlinux are relative to the filesystem containing extlinux.conf. If /boot is its own partition, then the above line would be LINUX /vmlinuz.old. I do not know if eclean-kernel can handle that, even if extlinux.py were to implement that logic.
3. Relative paths are relative to extlinux.conf. I do not know if eclean-kernel can handle that, even if extlinux.py were to implement that logic.

The config file is also a bit more complex than in my implementation: https://wiki.syslinux.org/wiki/index.php?title=Config#Location_and_name

The previous link also shows the EFI layout.