Protocol examples used to raise an error if the target was not set
and `idf.py build` command was used. This commit fix this error
and when IDF_TARGET is not set, ESP32 is selected as default target
linux/lwip: Wrap some IO posix functions
* to workaourd the FreeRTOS EINTR issue (when building without lwip)
* to correctly choose the sub-system based on fd (when building with
lwip) -- passing control to either linux/system or to lwip
This commit also addapts tapio-if to provide DHCP client by default and
configurable settings for static IP
