Recent dkms is broken
Using dkms to install hw-specific kernel modules, I get:
/usr/bin/dkms: line 864: /usr/lib/linux-kbuild-${$kernelver%.*}/scripts/sign-file: bad substitution
This is a fault in dkms itself, (or theoretically in bash).
It affects dkms 3.0.5-1 and 3.0.5-2. Downgrading to dkms 3.0.3 removes this problem.