[lkrn] Allow a single initrd to be passed to the booted kernel

[lkrn] Allow a single initrd to be passed to the booted kernel

Allow a single initrd image to be passed verbatim to the booted RISC-V
kernel, as a proof of concept.

We do not yet support reshuffling to make optimal use of available
memory, or dynamic construction of CPIO headers, but this is
sufficient to allow iPXE to start up the Fedora 42 kernel with its
matching initrd image.

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