samples: net: Create sample app for GSM modems

Sample app compiled with the GSM modem driver enabling PPP.

This sample was tested with a Reel Board UART_1 connected via the
external board/connector and a FONA 808 modem. Reel board specific
suppor is found in boards/.

Signed-off-by: Patrik Flykt <patrik.flykt@intel.com>