icetime: use $(PYTHON) in Makefile
icetime: use $(PYTHON) in Makefile

Allow the name of the Python interpreter to be changed.

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

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