Several Makefiles: Add .PHONY definitions to prevent 'clean up to date' message weirdness; Add default definitions for some commonly used things when 'make clean' is used without .config or Make.defs files installed.

Add a simple test of the scheduler instrumentation logic.  This initial version is still kind of buggy.

apps/modbus/nuttx/portserial.c:  Was not returning an error on failure to open the device driver.  From Olololshka kokoko

DHCPD no longer calls directly into the OS, but uses network IOCTL commands to modify the ARP table.