vyatta-cfg-system: move cpufrequtils from depends to recommends

Including the cpufrequtils package in the XenSource image causes a
number of fatal errors to be logged to the console on boot up, due to
missing devices on this platform.  Moving this package from depends to
recommends allows it to be excluded in these images, while still
maintaining it in non XenSource images.
0.20.44+vyos2+lithium5