package/pkg-python: <pkg>_SUBDIR cleanup

No functional change is brought by this modification.

This patch:
* removes redundant <pkg>_SRCDIR and <pkg>_BUILDDIR declarations
  (already defined in pkg-generic.mk)
* documents the usage of <pkg>_SUBDIR in the python-specific section of
  the manual.

Signed-off-by: Lionel Orry <lionel.orry@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>