boot.tmpSize option to adjust tmpfs size at /tmp

@CMCDragonkai
@CMCDragonkai You should be able to redefine the tmp.mount unit using the modules/boot/tmp.nix as a reference, just leave in 'size=${config.boot.tmpSize}' if still want to use use the size option.

I'm not sure what would happen if you defined your own tmp.mount and enabled the TmpOnTpmfs option.