Fix the nRF52840 dongle was not recognized when calling EnumSerialPorts

[General-zjs]

Fix the problem that the nRF52840 dongle was not recognized when calling the function 'EnumSerialPorts' in 'src\common\platform\win\serial_port_enum. cpp'

The issue is found: NordicSemiconductor/pc-ble-driver-py#64
https://devzone.nordicsemi.com/f/nordic-q-a/52531/the-issue-when-using-pc-ble-driver-py-for-nrf52840-dongle

Signed-off-by: h1562075881 1562075881@qq.com

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[General-zjs]

When I compile and run "examples port_enumeration main. c", the NRF52840 dongle cannot be recognized.
When I run "pc_ble_driver_py/examples/serial_port_enum. py" in Python, it also cannot be recognized.

[General-zjs]

[util.cmake]
When compiling connectivity, the default download path of nrf5 sdk source code is too long, which will cause compilation failure.
So change the default download path of nrf5 sdk source code to the pc-ble-driver home directory