tiny2313.upload.speed=9600
should be set to the right value and not commented out... I don't know the mega ISP from Arduino0018, but the IDE needs the upload.speed information.
and the bootloader is not specified... you write atmega8 instead of tiny2313...
So to sum it up: use this boards.txt entry:
tiny2313.name= tiny2313 based board (8MHz ext. crystal)
tiny2313.upload.protocol=stk500
tiny2313.upload.maximum_size=2048
tiny2313.upload.speed=9600
tiny2313.bootloader.low_fuses=0xEE
tiny2313.bootloader.high_fuses=0xDF
tiny2313.bootloader.path=tn2313bl
tiny2313.bootloader.file= tn2313bl.hex
tiny2313.bootloader.extended_fuses=0xFE
tiny2313.build.mcu=attiny2313
tiny2313.build.f_cpu=8000000L
tiny2313.build.core=attiny2313
and of course: make sure you have the files in the right directories (e.g. tn2313bl)