Improve radio RDS driver and framework

* Remove unused bits like the radio event and simplify basic
radio interface. It can be more self-contained with rds.h only
required by radio and tuner code.

* Add post-processing to text a-la Silicon Labs AN243. The chip's
error correction can only do so much; additional checks are highly
recommended. Simply testing for two identical messages in a row
is extremely effective and I've never seen corrupted text since
doing that, even with mediocre reception.

Groups segments must arrive in order, not randomly; logic change
only accepts them in order, starting at 0.

Time readout was made a bit better but really we'd need to use
verbose mode and ensure that no errors were seen during receiving
of time and more checks would be need to have a stable PI. The
text is the important bit anyway.

* Time out of stale text.

* Text is no longer updated until a complete group has been
received, as is specified in the standard. Perhaps go back to
scrolling text lines in the radio screen?

* Add proper character conversion to UTF-8. Only the default G0
table for the moment. The other two could be added in.

* Add variants "RDS_CFG_PROCESS" and "RDS_CFG_PUSH" to allow
the option for processed RDS data to be pushed to the driver and
still do proper post-processing (only text conversion for now for
the latter).

Change-Id: I4d83f8b2e89a209a5096d15ec266477318c66925

firmware/target/arm/as3525/fmradio-i2c-as3525.c

@@ -35,7 +35,6 @@
 #include "generic_i2c.h"
 #include "fmradio_i2c.h"
 #include "thread.h"
-#include "rds.h"
 
 #if     defined(SANSA_CLIP) || defined(SANSA_C200V2)
 #define I2C_SCL_GPIO(x) GPIOB_PIN(x)
@@ -203,13 +202,9 @@ void tuner_isr(void)
 /* Captures RDS data and processes it */
 static void NORETURN_ATTR rds_thread(void)
 {
-    uint16_t rds_data[4];
-
     while (true) {
         semaphore_wait(&rds_sema, TIMEOUT_BLOCK);
-        if (si4700_rds_read_raw(rds_data) && rds_process(rds_data)) {
-            si4700_rds_set_event();
-        }
+        si4700_rds_process();
     }
 }
 
@@ -233,7 +228,6 @@ void si4700_rds_powerup(bool on)
 void si4700_rds_init(void)
 {
     semaphore_init(&rds_sema, 1, 0);
-    rds_init();
     create_thread(rds_thread, rds_stack, sizeof(rds_stack), 0, "rds"
                   IF_PRIO(, PRIORITY_REALTIME) IF_COP(, CPU));
 }