Allow cow_spacesize to be set in profile
Allow cow_spacesize to be specified in profiledef.sh.
This eliminates the need to specify kernel options one at a time.
In the future, I think it would be nice to be able to specify the size as an argument of mkarchiso.
I added a cow_spacesize variable to mkarchiso and set the default value to 256M. (This is the value specified by mkinitcpio-archiso.)