Bluetooth support requires bluez-libs present, but they are only required
for the build, and don't seem to be needed to be present on the target.

There isn't any linking required to libbluetooth. It's only the bluetooth.h
header that is required for building BT support into Python.

For testing, this snippet was used from `Lib/test/test_socket.py` (inside
cpython):
```
def _have_socket_bluetooth():
    """Check whether AF_BLUETOOTH sockets are supported on this host."""
    try:
        # RFCOMM is supported by all platforms with bluetooth support. Windows
        # does not support omitting the protocol.
        s = socket.socket(socket.AF_BLUETOOTH, socket.SOCK_STREAM, socket.BTPROTO_RFCOMM)
    except (AttributeError, OSError):
        return False
    else:
        s.close()
    return True
```

Fixes: https://github.com/openwrt/packages/issues/16544

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

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>