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>
Fix idiotic typo in StrCaseCmp()

Signed-off-by: Peter Jones <pjones@redhat.com>
Audit fallback for return codes and alloc/free errors.

Signed-off-by: Peter Jones <pjones@redhat.com>
StrCaseCmp() should not be unsigned.

Signed-off-by: Peter Jones <pjones@redhat.com>
Don't leak a buffer.

Signed-off-by: Peter Jones <pjones@redhat.com>
Don't keep trying to add things if we're getting OUT_OF_RESOURCES

Signed-off-by: Peter Jones <pjones@redhat.com>
Use StrCaseCmp() to find boot.csv

Because who knows what somebody's fat implementation will return...

Signed-off-by: Peter Jones <pjones@redhat.com>
Explain what we're doing with GetInfo(), since the API is so bad.

Signed-off-by: Peter Jones <pjones@redhat.com>
vendor_cert_size's size in the binary should be 4, not -4.

The thing about subtraction is that the minuend needs to be before the
subtrahend in the text.

Signed-off-by: Peter Jones <pjones@redhat.com>
Move embedded certificates to their own section.

With this change, the embedded certificate and dbx lists (vendor_cert,
vendor_cert_size, vendor_dbx, and vendor_dbx_size) wind up being in a
section named .vendor_cert, and so will look something like:
------
fenchurch:~/devel/github.com/shim$ objdump -h shim.efi

shim.efi:     file format pei-x86-64

Sections:
Idx Name          Size      VMA               LMA               File off  Algn
  0 .eh_frame     000174a8  0000000000005000  0000000000005000  00000400  2**3
                  CONTENTS, ALLOC, LOAD, READONLY, DATA
  1 .text         000aa7e1  000000000001d000  000000000001d000  00017a00  2**4
                  CONTENTS, ALLOC, LOAD, READONLY, CODE
  2 .reloc        0000000a  00000000000c8000  00000000000c8000  000c2200  2**0
                  CONTENTS, ALLOC, LOAD, READONLY, DATA
  3 .data         00031228  00000000000c9000  00000000000c9000  000c2400  2**5
                  CONTENTS, ALLOC, LOAD, DATA
  4 .vendor_cert  00000375  00000000000fb000  00000000000fb000  000f3800  2**0
                  CONTENTS, READONLY
  5 .dynamic      000000f0  00000000000fc000  00000000000fc000  000f3c00  2**3
                  CONTENTS, ALLOC, LOAD, DATA
  6 .rela         0002afa8  00000000000fd000  00000000000fd000  000f3e00  2**3
                  CONTENTS, ALLOC, LOAD, READONLY, DATA
  7 .dynsym       0000f1f8  0000000000128000  0000000000128000  0011ee00  2**3
                  CONTENTS, ALLOC, LOAD, READONLY, DATA
------

This simplifies a security audit, because it means that different
versions of shim with substantially the same code with different keys
will be more easily comperable, and therefore logic differences may be
more easily identified.

This also means that if there's a trusted build you want to use, you can
remove the certificates, implant new ones, and have it signed, and the
code sections won't change.

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