python3: split python target configuration into own class

Setting _PYTHON_SYSCONFIGDATA_NAME in python3native class globally was
problematic as it was leaking into host python environment, which
was causing tracebacks depending on host distro and action
(typically anything involving importing sysconfig module).

The new class sets the variable only in specific tasks where it is needed,
and should be inherited explicitly:
- use python3native to run scripts with native python
- use python3targetconfig to run scripts with native python
if those scripts need to access target config data (such
as correct installation directories). This also adds a dependency
on target python, so should be used carefully to avoid lengthening builds.

(From OE-Core rev: 5a118d4e7985fa88f04c3611f8db813f0dafce75)

Signed-off-by: Alexander Kanavin <alex.kanavin@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/python3native.bbclass

@@ -17,8 +17,6 @@ export STAGING_LIBDIR
 export PYTHON_LIBRARY="${STAGING_LIBDIR}/lib${PYTHON_DIR}${PYTHON_ABI}.so"
 export PYTHON_INCLUDE_DIR="${STAGING_INCDIR}/${PYTHON_DIR}${PYTHON_ABI}"
 
-export _PYTHON_SYSCONFIGDATA_NAME="_sysconfigdata"
-
 # suppress host user's site-packages dirs.
 export PYTHONNOUSERSITE = "1"