Basically you need BOARD=NRF51822DK RELEASE=1 make (not NRF51822DK=1), then you should be ok.
and:
It's hex
You can't flash wirelessly if that's what you were asking... There's not space on the NRF51822 for a bootloader and Espruino, and even if there was the bootloaders are usually locked on the devcies. You need an nRF52832DK or NRF51822DK board, but then it's as easy as just copying the hex file to the new drive that appears when you plug the DK in
Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
Ahh, those instructions are old. There are some at https://github.com/espruino/Espruino/blob/master/README_Building.md
Basically you need
BOARD=NRF51822DK RELEASE=1 make
(notNRF51822DK=1
), then you should be ok.and: