package: Add cpulimit

Cpulimit is a tool which limits the CPU usage of a process (expressed in
percentage, not in CPU time). It is useful to control batch jobs, when
you don't want them to eat too many CPU cycles.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
[Peter: depend on BR2_USE_MMU, add pull request links to patches]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>