python3: make readline, gzip, bzip2 and xz really optional

python3: make readline, gzip, bzip2 and xz really optional

The BR2_PACKAGE_PYTHON3_{READLINE,ZLIB,BZIP2,XZ} 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_PYTHON3_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 python3.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>