icebox: use $(PYTHON) variable in Makefile
icebox: use $(PYTHON) variable in Makefile

Allow `python` to be provided by an interpreter other than `python3`.

Signed-off-by: Sean Cross <d7e19930cc1f42c2d0781f4d9e6f1fe5891bf9cf@xobs.io>

SymbiFlow/icestorm,SymbiFlow/icestorm,SymbiFlow/icestorm,SymbiFlow/icestorm