Detect and log when we're EFI-booting a non-EFI-capable binary

[danderson]

EFI iPXE can only boot EFI binaries. This generally works because linux has CONFIG_EFI_STUB, a cool hack that makes bzImage kernels multi-format, executable as either bzImage or EFI applications.

If you try booting a kernel that doesn't have CONFIG_EFI_STUB, at best iPXE will fail because of a format error. At worst, it will chainload anyway and hang the machine.

It's possible to detect whether an image is a proper EFI application by inspecting the first few dozen bytes of the kernel. Pixiecore should do an on-the-fly inspection of the kernel as it's serving the image, and log a loud warning if it's serving a non-EFI image to an EFI client.

[danderson]

Some reading about CONFIG_EFI_STUB: https://lwn.net/Articles/632528/