- working bootloader version
- changed the advertising name to STK-DFU


how to create bootloader file for firmware update via DFU

1) command to create bootloader zip file
   nrfutil pkg generate --hw-version 51 --bootloader-version 3 --bootloader nrf51422_xxac_s130.hex --sd-req 0x87 --key-file private.key fw_bootloader.zip

Note: bootloader version has to be increased by 1 to update the bootloader
ex: if existing bootloader on the device is version 1, then while creating the zip file, the bootloader version should be more than the existing version