x86/boot/Documentation: Prefix hexadecimal literals with 0x

x86/boot/Documentation: Prefix hexadecimal literals with 0x

The x86 bootloader ID specification text uses hexadecimal
values without a 0x prefix:

        D  kexec-tools
        E  Extended (see ext_loader_type)
        F  Special (0xFF = undefined)
        10 Reserved
        11 Minimal Linux Bootloader
           <http://sebastian-plotz.blogspot.de>
        12 OVMF UEFI virtualization stack
        13 barebox

Which beyond the ambiguity of '13' in isolation, also
made me fail a grep -wi '0xd' when I was looking for
the kexec bootloader ID definition and caused quite
a bit of head-scratching before I found out why it
didn't show up.

Furthermore, the actual explanatory text uses the 0x
prefix:

  For boot loader IDs above T = 0xD, write T = 0xE to this field and
  write the extended ID minus 0x10 to the ext_loader_type field.
  Similarly, the ext_loader_ver field can be used to provide more than
  four bits for the bootloader version.

So make it all both unambiguous, easy to grep and consistent
across the entire documentation by prefixing the IDs with 0x.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: linux-kernel@vger.kernel.org