python: make hashlib and readline modules really optional

python: make hashlib and readline modules really optional

The BR2_PACKAGE_PYTHON_{READLINE,HASHLIB} options were so far only
bringing in the necessary dependencies, relying on the Python build
system to automatically detect them.

However, this means that even if one of those option was disabled, if
their dependency was found, Python would build the corresponding module,
which is really not what the user would expect.

For example, if you have:

   BR2_PACKAGE_READLINE=y
   # BR2_PACKAGE_PYTHON_READLINE is not set

Then you would still get the readline Python module built and installed.

This commit fixes that by adding new --{enable,disable} options, and use
them in python.mk.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>