fix(build_image): Switch to SYSLINUX for our EFI bootloader.

Now with SYSLINUX 6 we can use the same bootloader on EFI and BIOS
systems. This replaces our previous reliance on building default kernel
options into the kernel image itself.
Merge pull request #199 from marineam/syslinux

fix(build_image): Switch to SYSLINUX for our EFI bootloader.