When creating a custom bootloader for buttonless DFU, it is surprising to see that nrf_dfu.c not compile without defining button related function.
The warning
nRF5_SDK_14.2.0_17b948a/components/libraries/bootloader/dfu/nrf_dfu.c:81: undefined reference to `nrf_dfu_button_enter_check'
was thrown, and the make fails.
This is with nRF5 SDK 14.2.0.