packge/pkg-python: drop custom environment variables

Many moons ago, in the dark ages of Python, cross compiles were largely
unsupported. In these before-times, a patchset used by PtxDist [0] [1]
was adapted to help make cross compiles work.

The patchset did a number of things but mainly:
 1) used a build-machine compatible python interpreter for certain
    stages of the target Python build process
 2) made adjustments to certain files to make decisions based on values
    set in environment variables instead of the path of the executing
    Python interpreter.

Since the path of the interpreter that was build machine compatible was
outside of the target build directory, the code that made assumptions
about the location of headers and library paths being relative to the
interpreter path needed to be adjusted, hence them being driven via
environment variables.

The patchset worked by replacing the executable path to be the sysroot
which included the python headers and libraries.

A number of issues regarding cross compilation [2] [3] [4] have since
been closed since the introduction of this patchset and cross builds
became much better supported starting in Python v3.3.1.

New logic primarily uses the _PYTHON_PROJECT_BASE env variable [5] [6].

When set properly, this drives a few things:
  * flags a cross compile environment
  * sysconfig.is_python_build = True which triggers:
    * altered paths for finding the Makefile and config.h
    * altered sysconfig.get_config_var("srcdir")

When migrating to Python 3.4, PtxDist reworked their patchset to use
the standard environment variables for their cross compiles [7].

The distutils module was a primary consumer of the custom variables from
the previous patchset, however, that module is deprecated and packages
cannot target it as of 09de823c.

Package builds and unit tests seem to work without using these variables
being set, implying they can likely be dropped. Packages that still use
distutils should be updated to reflect its removal in 3.12.

Once these custom variables are removed, the following Python3 patches
which leverage them can be dropped:
  0004-Adjust-library-header-paths-for-cross-compilation
  0009-Do-not-adjust-the-shebang-of-Python-scripts-for-cros

[0]: eef994411c
[1]: 6c79cb5ac3
[2]: https://github.com/python/cpython/issues/48004
[3]: https://github.com/python/cpython/issues/58538
[4]: https://github.com/python/cpython/issues/59689
[5]: 7e6c2e2cc4
[6]: 9731330d6f
[7]: 638a024500

Signed-off-by: Vincent Fazio <vfazio@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/pkg-python.mk

@@ -41,10 +41,7 @@ PKG_PYTHON_ENV = \
 	PATH=$(BR_PATH) \
 	$(TARGET_CONFIGURE_OPTS) \
 	PYTHONPATH="$(PYTHON3_PATH)" \
-	PYTHONNOUSERSITE=1 \
-	_python_sysroot=$(STAGING_DIR) \
-	_python_prefix=/usr \
-	_python_exec_prefix=/usr
+	PYTHONNOUSERSITE=1
 
 # Host python packages
 HOST_PKG_PYTHON_ENV = \