build: Add IRQSOFF and PREEMPT TRACER kernel config option

This adds the CONFIG_IRQSOFF_TRACER and the CONFIG_PREEMPT_TRACER kernel
configuration option to the OpenWrt menu. This can be used to debug
latencies in the system.
The CONFIG_PREEMPT_TRACER option needs the CONFIG_PREEMPT option which is
supposed to be used for Low-Latency Desktop and not used by many targets
in OpenWrt.

The help text is copied from the Linux kernel Kconfig.

Signed-off-by: Hauke Mehrtens <hmehrtens@maxlinear.com>