efi: libstub: Undeprecate the command line initrd loader

The initrd= command line loader can be useful for development, but it
was limited to loading files from the same file system as the loaded
kernel (and it didn't work on x86 mixed mode).

As both issues have been fixed, and the initrd= can now be used with
files residing on any simple file system exposed by the EFI firmware,
let's permit it to be enabled on RISC-V and LoongArch, which did not
support it up to this point.

Note that LoadFile2 remains the preferred option, as it is much simpler
to use and implement, but generic loaders (including the UEFI shell) may
not implement this so there, initrd= can now be used as well (if enabled
in the build)

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>