Arduino bootloader

As of arduino-1.8.5 the bootloader used for arduino uno (as specified in boards.txt uno.bootloader.file=optiboot/optiboot_atmega328.hex) is optiboot

The file location is
~/.arduino15/packages/arduino/hardware/avr/1.6.19/bootloaders/optiboot/

The Makefile uses -mshort-calls which is not defined anymore as of avr-gcc 4.8.2.
Use -mrelax instead

Connections:
https://www.arduino.cc/en/Tutorial/ArduinoISP

Sources:

https://www.avrfreaks.net/forum/where-has-mshort-calls-compiler-option-gone