rds: implement more robust PS/RT parsing

rds: implement more robust PS/RT parsing

Our current PS and RT parser is very strict: it requires
all segments to be received in order. This is too strong
of an assumption even when the reception in good, particularly
for long RT messages. This parser handles segments in any order
and completes a message only when all segments are present. To
avoid keeping obsolete segments, each segment has an associated
timeout and is trashed if not updated for a certain period.

Change-Id: I79841bb5ab1f838c7702d8281044e226f6510199
Reviewed-on: http://gerrit.rockbox.org/237
Tested-by: Bertrik Sikken <bertrik@sikken.nl>
Reviewed-by: Bertrik Sikken <bertrik@sikken.nl>