Fix kernel version selection using build machine rather than target

This fix uses the 'setup.cfg' file used by setuptools. It reads an
Adafruit-specific section that is ignored by setuptools.

The reason for using the config file is that command-line parameters can't
be passed to setup.py, as the command-line is ideally opaque to the
installer. 'setup.cfg' was added to setuptools as a useful middle-ground
between modifying 'setup.py' and the command-line. Now, library build
systems can modify a config file instead of the source directly.

Fixes adafruit#256

Adafruit_Config.py

@@ -0,0 +1,80 @@
+# Adafruit Setuptools Configuration Module
+# SPDX-License-Identifier: MIT
+
+try:
+  import configparser
+except ImportError:
+  # Python 2 compatibility
+  import ConfigParser as configparser
+
+class Adafruit_Config:
+  """
+  Class to obtain configuration provided by the setup.cfg configuration file
+  used by setuptools.
+
+  Adafruit-specific information is provided under an [adafruit] section.
+
+  The reason for this interface is that the command-line to the setup.py
+  script is out of our control and entirely up to the installer. The
+  setup.cfg file was added to setuptools as a useful middle-ground between
+  modifying the setup script and the command-line to the setup script.
+  """
+
+  #############################################################################
+  # Parameters
+  #############################################################################
+
+  # Setuptools config file
+  SETUPTOOLS_CONFIG_FILE = 'setup.cfg'
+
+  # Section name in cfg file
+  SECTION_NAME = 'adafruit'
+
+  # Key name for specifying kernel >= 4.1.0 targets. Value must be 'true' to
+  # target the newer kernel interface or 'false' to target the older one.
+  KERNEL41_KEY = 'kernel41'
+
+  #############################################################################
+  # Interface
+  #############################################################################
+
+  def __init__(self):
+    """
+    Initialize the configuration object.
+    """
+    self._metadata = self.metadata_from_setupcfg(self.SECTION_NAME)
+
+  def is_kernel41(self):
+    """
+    Return True if we are targeting a kernel >= 4.1.0, or False otherwise.
+    Defaults to True if unknown.
+    """
+    return False if self.get_cfg_value(self.KERNEL41_KEY) == 'false' else True
+
+  #############################################################################
+  # Helpers
+  #############################################################################
+
+  def get_cfg_value(self, key):
+    """
+    Get a value associated with a key in the Adafruit section. Returns the
+    value, or None if unknown.
+    """
+    return self._metadata.get(key, None)
+
+  @classmethod
+  def metadata_from_setupcfg(cls, section_name):
+    """
+    Read the [adafruit] section of setup.cfg and return it as a dict.
+    """
+    cfgparser = configparser.ConfigParser()
+    cfgparser.read(cls.get_cfg_fname())
+
+    return dict(cfgparser.items(section_name))
+
+  @classmethod
+  def get_cfg_fname(cls):
+    """
+    Get the name of the setuptools cfg file.
+    """
+    return cls.SETUPTOOLS_CONFIG_FILE

setup.cfg

@@ -0,0 +1,6 @@
+# Adafruit Setuptools Configuration Interface
+[adafruit]
+
+# Set this to 'true' if targeting a kernel >= 4.1.0 to use the new kernel
+# interface, or 'false' to use the old interface.
+kernel41 = true

setup.py

@@ -1,3 +1,5 @@
+from Adafruit_Config import Adafruit_Config
+
 try:
     from overlays import builder
     builder.compile()
@@ -7,16 +9,14 @@
 
 import distribute_setup
 import io
-import sys
-import platform
 distribute_setup.use_setuptools()
 from setuptools import setup, Extension, find_packages
 
 open_as_utf8 = lambda x: io.open(x, encoding='utf-8')
 
-kernel = platform.release()
+config = Adafruit_Config()
 
-if kernel >= '4.1.0':
+if config.is_kernel41():
     kernel41 = [('BBBVERSION41', None)]
 else:
     kernel41 = None