Casper doesn't properly support NFSOPTS

From https://www.kernel.org/doc/Documentation/filesystems/nfs/nfsroot.txt:
nfsroot=[<server-ip>:]<root-dir>[,<nfs-options>]

NFSOPTS are properly supported by initramfs-tools/scripts/nfs:
if [ "${NFSROOT#*,}" != "${NFSROOT}" ]; then
    NFSOPTS="-o ${NFSROOT#*,}"
fi

But casper doesn't support them properly like this.
Casper assumes that the scripts/nfs code has already ran and set NFSOPTS, which it didn't.
So the only way to set NFSOPTS with casper is to actually pass NFSOPTS=xxx in the kernel cmdline; the kernel itself then passes this as an init parameter.

This behavior is wrong and undocumented, NFSOPTS isn't a kernel parameter but an internal variable of initramfs-tools/scripts/nfs, which isn't set when casper runs, and casper needs to set it by doing an "if" like the one above ^.