move standalone core generation to litedram package and make it usable externally

When LiteDRAM is installed, standalone core can now be generated with "litedram_gen config.yml"

setup.py

@@ -33,4 +33,9 @@ setup(
     ],
     packages=find_packages(exclude=("test*", "sim*", "doc*", "examples*")),
     include_package_data=True,
+    entry_points={
+        "console_scripts": [
+            "litedram_gen=litedram.gen:main",
+        ],
+    },
 )

test/test_examples.py

@@ -8,7 +8,7 @@ import os
 def build_config(name):
     errors = 0
     os.system("rm -rf examples/build")
-    os.system("cd examples && python3 litedram_gen.py {}.yml".format(name))
+    os.system("cd examples && litedram_gen {}.yml".format(name))
     errors += not os.path.isfile("examples/build/gateware/litedram_core.v")
     os.system("rm -rf examples/build")
     return errors