This adds a config option PYTHON3_HOST_PIP_CACHE_WORLD_READABLE; if
enabled, chmod will be run after pip install to make all
files/directories in the host pip cache world-readable.

Supersedes https://github.com/openwrt/packages/pull/13012.

Signed-off-by: Jeffery To <jeffery.to@gmail.com>

It was reported via
https://github.com/openwrt/packages/pull/5122#issuecomment-347395472
that if bluez-libs is selected as an installable package,
then the error below will show up:
```
 * satisfy_dependencies_for: Cannot satisfy the following dependencies for python-light:
 *	bluez-libs *
 * opkg_install_cmd: Cannot install package python-light.
```

This looks like a limitation in the design of package deps,
and maybe a misuse of conditional deps (i.e. PACKAGE_bluez-libs:bluez-libs).

So, to fix this, an idea we're adding an extra symbol
that enfoces installation of bluez-libs if selected.

We also need to add a way to disable bluetooth build
if PYTHON(3)_BLUETOOTH_SUPPORT is de-selected.
Otherwise, bluetooth is installed and the socket
module is broken due to linker errors.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>