lxbuildenv: fix generation of initial python

It still doesn't work (owing to some bit rot), but it's better now.

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

lxbuildenv.py

@@ -178,6 +178,9 @@ def check_yosys(args):
 def check_arachne(args):
     return check_cmd(args, "arachne-pnr")
 
+def check_icestorm(args):
+    return check_cmd(args, "icepack") and check_cmd(args, "nextpnr-ice40")
+
 dependency_checkers = {
     'python': check_python_version,
     'vivado': check_vivado,
@@ -185,6 +188,7 @@ dependency_checkers = {
     'riscv': check_riscv,
     'yosys': check_yosys,
     'arachne-pnr': check_arachne,
+    'icestorm': check_icestorm,
 }
 
 # Validate that the required dependencies (Vivado, compilers, etc.)
@@ -361,7 +365,11 @@ import lxbuildenv
 #pylint:disable=E1101
 
 from migen import *
-from litex.build.generic_platform import *
+from litex.build.xilinx import VivadoProgrammer, XilinxPlatform
+from litex.build.generic_platform import Pins, IOStandard
+from litex.soc.integration import SoCSDRAM
+from litex.soc.integration.builder import Builder
+from litex.soc.integration.soc_core import csr_map_update
 
 _io = [
     ("clk50", 0, Pins("J19"), IOStandard("LVCMOS33")),