Minor fixups following #23 / #56

- Update the codegen class docstring for LTO.
- Simplify / correct some logic in `_readenv()` (`value.lower()` could
  never be `"True"`, only `"true"`.
- Simplify additional flags and linker checks.
- Setting `self._linker.complete` in `complete()` is unnecessary, as
  calling `get_linked_cubin()` sets the link as complete already.

numba_cuda/numba/cuda/codegen.py

@@ -70,14 +70,16 @@ def __init__(
     ):
         """
         codegen:
-        Codegen object.
+            Codegen object.
         name:
             Name of the function in the source.
         entry_name:
             Name of the kernel function in the binary, if this is a global
             kernel and not a device function.
         max_registers:
             The maximum register usage to aim for when linking.
+        lto:
+            Whether to enable link-time optimization.
         nvvm_options:
                 Dict of options to pass to NVVM.
         """

numba_cuda/numba/cuda/cudadrv/driver.py

@@ -64,7 +64,7 @@ def _readenv(name, ctor, default):
         return default() if callable(default) else default
     try:
         if ctor is bool:
-            return bool(value.lower() in {'1', "True"})
+            return value.lower() in {'1', "true"}
         return ctor(value)
     except Exception:
         warnings.warn(
@@ -2631,7 +2631,7 @@ def new(cls,
 
         if linker is PyNvJitLinker:
             return linker(max_registers, lineinfo, cc, lto, additional_flags)
-        elif additional_flags is not None or lto is True:
+        elif additional_flags or lto:
             raise ValueError("LTO and additional flags require PyNvJitLinker")
         else:
             return linker(max_registers, lineinfo, cc)
@@ -3088,9 +3088,7 @@ def add_data(self, data, kind, name):
 
     def complete(self):
         try:
-            cubin = self._linker.get_linked_cubin()
-            self._linker._complete = True
-            return cubin
+            return self._linker.get_linked_cubin()
         except NvJitLinkError as e:
             raise LinkerError from e