litex_setup: Use subprocess so failures are noticed.

os.system doesn't report if any of the commands fail. This means that if
something goes wrong it happily reports success making it hard to debug
issues.

litex_setup.py

@@ -2,6 +2,7 @@
 
 import os
 import sys
+import subprocess
 from collections import OrderedDict
 
 
@@ -46,7 +47,9 @@ if "init" in sys.argv[1:]:
         print("[cloning " + name + "]...")
         full_url = url + name
         opts = "--recursive" if need_recursive else ""
-        os.system("git clone " + full_url + " " + opts)
+        subprocess.check_call(
+            "git clone " + full_url + " " + opts,
+            shell=True)
 
 if "install" in sys.argv[1:]:
     for name in repos.keys():
@@ -56,13 +59,19 @@ if "install" in sys.argv[1:]:
         if need_develop:
             os.chdir(os.path.join(current_path, name))
             if "--user" in sys.argv[1:]:
-                os.system("python3 setup.py develop --user")
+                subprocess.check_call(
+                    "python3 setup.py develop --user",
+                    shell=True)
             else:
-                os.system("python3 setup.py develop")
+                subprocess.check_call(
+                    "python3 setup.py develop",
+                    shell=True)
 
 if "update" in sys.argv[1:]:
     for name in repos.keys():
         # update
         print("[updating " + name + "]...")
         os.chdir(os.path.join(current_path, name))
-        os.system("git pull")
+        subprocess.check_call(
+            "git pull",
+            shell=True)