drivers: initial implementation of sx127x radio (LoRa mode only)

cpu/cc2538/radio/cc2538_rf_getset.c

drivers/Makefile.dep

drivers/Makefile.include

drivers/include/net/netdev.h

drivers/include/sx127x.h

drivers/sx127x/Makefile

drivers/sx127x/include/sx127x_internal.h

drivers/sx127x/include/sx127x_netdev.h

drivers/sx127x/include/sx127x_params.h

drivers/sx127x/include/sx127x_registers.h

drivers/sx127x/sx127x.c

drivers/sx127x/sx127x_getset.c

drivers/sx127x/sx127x_internal.c

drivers/sx127x/sx127x_netdev.c

makefiles/pseudomodules.inc.mk