Add a fallback loader for when shim is invoked as BOOTX64.EFI

Add a fallback loader for when shim is invoked as BOOTX64.EFI

If shim is invoked as \EFI\BOOT\BOOT*.EFI and a file exists named
\EFI\BOOT\FALLBACK.EFI, try it instead of our second stage.  So don't
put fallback.efi on your install media in \EFI\BOOT, because that won't
do whatever it is you're hoping for, unless you're hoping not to start
the installer.

So here's the process for using this:
in /EFI/fedora/ (or whichever directory you happen to own), you put:
  shim.efi
  grub.efi
  boot.csv - format is: shim.efi,Nice Label,cmdline arguments,comments
           - filenames refer only to files in this directory, with no
	     leading characters such as L"./" or L"/EFI/fedora/"
           - note that while this is CSV, the character encoding is
	     UCS-2

and if /EFI/BOOT/BOOTX64.EFI doesn't already exist, then in /EFI/BOOT:
  shim.efi as BOOTX64.EFI
  fallback.efi

Signed-off-by: Peter Jones <pjones@redhat.com>