Measure response time to GPIO events sent by a remote Zephyr-based
latency monitor (latmon). This monitor collects then sends the latency
data over a network connection to the latmus front-end which displays
the results received:

[Linux device under test running latmus]  <--+
                                             |
        ^                 | (GPIO ack)       |
        |                 |                  |  TCP/IP network connection
        |                 |                  |  (control & data samples)
        | (GPIO pulse)    v                  |
                                             |
[Zephyr-based device running latmon]      <--+

The generic latency monitor code running the measurement loop is
available from the zephyr/ directory. This support comes with a Zephyr
device tree patch for enabling this monitor on NXP's FRDM-K64F
development board.