To upload a new sketch on ATtiny25 by using an Arduino Mega 2560 as ISP, boards.txt should include
attiny25.name=ATtiny25 attiny25.upload.using=arduino:arduinoisp attiny25.bootloader.low_fuses=0x62 attiny25.bootloader.high_fuses=0xdf attiny25.bootloader.extended_fuses=0xff attiny25.upload.maximum_size=2048 attiny25.build.mcu=attiny25 attiny25.build.f_cpu=1000000L attiny25.build.core=arduino:arduino attiny25.build.variant=tiny8
Arduino Mega 2560 has a built-in auto reset which will prevent the upload. To disable this auto reset, add a 10uF capacitor between RESET and GND on the Arduino board. Remember that, if you are using a electrolytic capacitor, the anode is connected to GND.