OvmfPkg: add the 'initrd' dynamic shell command

OvmfPkg: add the 'initrd' dynamic shell command

Add the 'initrd' dynamic shell command to the build so we can load
Linux initrds straight from the shell using the new generic protocol,
which does not rely on initrd= being passed on the command line.

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2564
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>