[efi] Support the initrd autodetection mechanism in newer Linux kernels

Linux 5.7 added the ability to autodetect an initrd by searching for a
handle via a fixed vendor-specific "Linux initrd device path" and then
locating and using the EFI_LOAD_FILE2_PROTOCOL instance on that
handle.

This maps quite naturally onto our existing concept of a "magic
initrd" as introduced for EFI in commit e5f0255 ("[efi] Provide an
"initrd.magic" file for use by UEFI kernels").

Add an EFI_LOAD_FILE2_PROTOCOL instance to our EFI virtual files
(backed by simply calling the existing EFI_SIMPLE_FILE_SYSTEM_PROTOCOL
method to read from the file), and install the protocol instance for
the "initrd.magic" virtual file onto a new device handle that also
provides the Linux initrd device path.

The design choice in Linux of using a single fixed device path makes
this unfortunately messy to support, since device paths must be unique
within a system.  When multiple bootloaders are used (e.g. GRUB
loading iPXE loading Linux) then only one bootloader can ever install
the device path onto a handle.  Subsequent bootloaders must locate the
existing handle and replace the load file protocol instance with their
own.

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

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