Building Espruino for Puck

try make clean;DFU_UPDATE_BUILD=1 PUCKJS=1 RELEASE=1 make - that's what you need I think (but you'll need to pip install nrfutils)

The normal build will try and merge with the bootloader, which needs building with make clean;BOOTLOADER=1 PUCKJS=1 RELEASE=1 make

However that merged hex file is only really any good if you've got a debugger connected to the Puck - for wireless firmware updates you want the DFU_UPDATE_BUILD bit

Hope that helps!