mbed deploy
mbed config root .
# set the desired compiler
mbed toolchain GCC_ARM
# set the desired board
mbed target NUCLEO_L073RZ
# set-up the compiler path if needed
mbed config -G GCC_ARM_PATH /opt/gcc-arm-none-eabi-7-2017-q4-major/bin
cp mbed_app.json.sample mbed_app.json
# modify lora.device-eui, lora.application-eui and lora.application-key in mbed_app.json
mbed compile --profile minimal-printf/profiles/release.json

Then you can use stm32flash or - may be better - a stlink interface to copy the generated .bin file onto the RAK811.

Notes

Tested boards

MTB_RAK811
NUCLEO_L073RZ + SX1272MB2xAS

use STLink from a Nucleo board with the RAK811 board

From STlink CN4 Connector to RAK811 "shortest" connector (pin 1 at the same side of antenna connectors)

2 -> 3 (CLK)
3 -> 1 (GND)
4 -> 4 (DATA)
5 -> 2 (RST)

rewire STLink on a cutted Nucleo board

From STlink CN4 Connector to Nucleo CN7

2 -> 15 (CLK)
3 -> 19,20,22 (GND)
4 -> 13 (DATA)
5 -> 14 (RST)

From STLink CN3 Connector to Nucleo CN7 (stdio through USB)

rx -> 1 (USART4_TX)
tx -> 2 (USART4_RX)