Add LIBS input property to GENERATE_ARDUINO_EXAMPLE

[ryanpfeeley]

This allows users to add static libraries to fix linking errors with generate_arduino_example that can sometimes occur.

Some arduino examples with multiple dependencies give linking errors (undefined references) when built with the generate_arduino_example command. For example,

generate_arduino_example(cardinfo-ex 
      LIBRARY SD
      EXAMPLE CardInfo
      BOARD pro) 

Will fail to properly link with the SPI library. There is possibly a way to fix this under-the-hood with improved dependency discovery. I don't know where to start with this. As work-around that is perfectly consistent with the other arduino-cmake function, this change adds the LIBS input argument to generate_arduino_example. This works like the existing LIBS inputs on generate_arduino_firmware and generate_arduino_library

The user can now use generate_arduino_library to create a static library for SPI, and then build the example with

generate_arduino_example(cardinfo-ex 
      LIBRARY SD
      EXAMPLE CardInfo
      LIBS SPI #SPI target created with generate_arduino_library
      BOARD pro) 

The cmake source code for the generate_arduino_example already handled
this input since it contains list(APPEND ALL_LIBS ${CORE_LIB} ${INPUT_LIBS}), so no changes were necessary for the function itself.

Hopefully this is helpful.