libserialport: drop untrue statement from Config.in help

According to http://sigrok.org/wiki/Libserialport

:

  Note: While libserialport is hosted on sigrok.org (and sigrok
  uses libserialport), this is a completely independent library
  that can be used by other projects as well. The libserialport
  library does not depend on any sigrok related libraries or
  projects.

Drop the fragment about being a part of the sigrok suite and
extend the help text.

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