package/linux-tools: Allow installation of perf scripts

In order to run the full gamut of "perf tests" installing the perf
scripts is required, otherwise we run into errors similar to those:

	failed to open shell test directory: /usr/libexec/perf-core/tests/shell

Make this possible by introducing a new option allowing us to install
perf scripts.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>