Splitting out UEFI bootloader code from kernel

Now the bootloader should be responsible for all initial setup,
loading the kernel, and then handing off to the kernel with
proper data in place.

src/boot/loader.h

@@ -0,0 +1,8 @@
+#pragma once
+#include <efi.h>
+
+#ifndef KERNEL_FILENAME
+#define KERNEL_FILENAME L"kernel.bin"
+#endif
+
+EFI_STATUS loader_load_kernel(void **kernel_image, UINT64 *len);