libsigrok: don't select libserialport in Config.in

Libsigrok can be built without libserialport. Don't select it
in Config.in from both libsigrok and sigrok-cli and add a check
to libsigrok.mk to determine whether libserialport should be
enabled.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>