CMake library for WiFi

[glebm]

Currently it is possible to use Portduino WiFi from CMake by defining the library manually like so:

FetchContent_Declare(PortduinoWifi
  URL https://github.com/meshtastic/WiFi/archive/b885b9595d54ee6eae59696eeae98f631eb27a23.tar.gz
  URL_HASH MD5=af81fff3f7cd2127cf54d5cfc13a95ef
  EXCLUDE_FROM_ALL)
FetchContent_MakeAvailable(PortduinoWifi)

add_library(PortduinoWifi STATIC
  ${portduinowifi_SOURCE_DIR}/src/WiFiClient.cpp
  ${portduinowifi_SOURCE_DIR}/src/WiFi.cpp
  ${portduinowifi_SOURCE_DIR}/src/WiFiServer.cpp
  ${portduinowifi_SOURCE_DIR}/src/WiFiUdp.cpp
)
target_include_directories(PortduinoWifi PUBLIC ${portduinowifi_SOURCE_DIR}/src)
target_link_libraries(PortduinoWifi PUBLIC Portduino::Portduino)

(the Portduino::Portduino target ehre is from #34)

It'd be better if https://github.com/meshtastic/WiFi provided a target instead.

Posting here because https://github.com/meshtastic/WiFi does not have Issues enabled.