boot.img and core.img were compiled from git://git.savannah.gnu.org/grub.git,
commit 72ec399ad8d6348b6c74ea63d80c79784c8b84ae, on a Debian 7.7.0 x64 system.
This was done following the guide from:
http://pete.akeo.ie/2014/05/compiling-and-installing-grub2-for.html.

boot.img has been modified to nop the jump @ 0x66 as per grub2's setup.c comments:

/* If DEST_DRIVE is a hard disk, enable the workaround, which is
   for buggy BIOSes which don't pass boot drive correctly. Instead,
   they pass 0x00 or 0x01 even when booted from 0x80.  */
   
Note that, for convenience reasons, the content of boot.img is *not* the one that
Rufus processes when writing the MBR.
Instead, the byte array from src/ms-sys/inc/mbr_grub2.h (whose content is identical)
is what Rufus uses. If you modify these files, be mindful that you may also need
to update the array in mbr_grub2.h.