package/gpsd: python support needs pyserial

Usually, ubxtool, a python-based tool to drive u-blox devices, connects
to a gpsd instance and delegates to it the responsibility to write to
and read from the actual device. This is sane, because a serial device
can only be opened once, and if gpsd is running, it has that device
open.

However, in some cases, ubxtool can be used to directly talk to the
device, to pre-configure it before gpsd runs, or even in the absence of
gpsd altogether. This is not used very often, except when setting up an
RTK base, where gpsd is not needed.

In that case, ubxtool will directly talk to the serial device. It uses
the pyserial python module. Since this is not the traditional way to
talk to the device, failure to import the module is ignored, and the
error reporting is deferred until it is actually needed, which is why we
did not catch the issue earlier. See [1] and [2].

Fixes: f3ef0723cf (package/gpsd: enable python support and modules)

[1] https://gitlab.com/gpsd/gpsd/-/blob/release-3.25/clients/ubxtool.py.in#L47
[2] https://gitlab.com/gpsd/gpsd/-/blob/release-3.25/gps/gps.py.in#L36

Signed-off-by: Yann E. MORIN <yann.morin@orange.com>
Cc: Bernd Kuhls <bernd@kuhls.net>
[Julien: add link to described code portion]
Signed-off-by: Julien Olivain <ju.o@free.fr>
(cherry picked from commit 5d2f3737a1)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/gpsd/Config.in

@@ -80,6 +80,7 @@ config BR2_PACKAGE_GPSD_PYTHON
 	bool "build Python support and modules"
 	depends on BR2_USE_WCHAR # python3
 	select BR2_PACKAGE_PYTHON3
+	select BR2_PACKAGE_PYTHON_SERIAL # runtime
 	help
 	  Python libraries and tools for the gpsd service daemon
 	  including gpsfake test harness.