cmake: Have gperf be opt-out instead of mandatory

gperf is only used when CONFIG_USERSPACE is enabled. Users that use
arch's that don't support CONFIG_USERSPACE, or happen to not never
enable CONFIG_USERSPACE should not be artificially required to install
gperf.

This change makes gperf optional by moving it's presence-check to it's
usage.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>
2 files changed