[efi] Provide an "initrd.magic" file for use by UEFI kernels

[efi] Provide an "initrd.magic" file for use by UEFI kernels

Provide a file "initrd.magic" via the EFI_SIMPLE_FILE_SYSTEM_PROTOCOL
that contains the initrd file as constructed for BIOS bzImage kernels
(including injected files with CPIO headers constructed by iPXE).

This allows BIOS and UEFI kernels to obtain the exact same initramfs
image, by adding "initrd=initrd.magic" to the kernel command line.
For example:

  #!ipxe
  kernel boot/vmlinuz initrd=initrd.magic
  initrd boot/initrd.img
  initrd boot/modules/e1000.ko      /lib/modules/e1000.ko
  initrd boot/modules/af_packet.ko  /lib/modules/af_packet.ko
  boot

Do not include the "initrd.magic" file within the root directory
listing, since doing so would break software such as wimboot that
processes all files within the root directory.

Signed-off-by: Michael Brown <mcb30@ipxe.org>