Enable extlinux support for (non-Ubuntu) Debian platforms

Extlinux bootloader configuration in "vm" element requires vmlinuz and
initrd.img files in /boot/ with "generic" suffix. When these don't exist,
bootloader configuration should fall back to using whatever does exist.

Change-Id: I79d426e8f008e9f23db899f0b8f8cf2f23643e28
Merge "Enable extlinux support for (non-Ubuntu) Debian platforms"