I had to make some modifications to Christo's boards.txt code to make it work with Arduino IDE version 1.82.
atmega8u.name=ATMEGA8
atmega8u.upload.tool=avrdude
atmega8u.upload.protocol=arduino
atmega8u.upload.maximum_size=7680
atmega8u.upload.maximum_data_size=1024
atmega8u.upload.speed=115200
atmega8u.bootloader.low_fuses=0xff
atmega8u.bootloader.high_fuses=0xcc
atmega8u.bootloader.file=optiboot/optiboot_atmega8.hex
atmega8u.bootloader.extended_fuses=
atmega8u.bootloader.unlock_bits=0x3F
atmega8u.bootloader.lock_bits=0x0F
atmega8u.bootloader.tool=avrdude
atmega8u.build.board=AVR_NG
atmega8u.build.mcu=atmega8
atmega8u.build.f_cpu=16000000L
atmega8u.build.core=arduino
atmega8u.build.variant=standard