Changes

codec: add some constant frames for HR and EFR

With all 3 classic GSM codecs (FR, HR and EFR) sometimes a need
arises for a constant fill frame, to be inserted when no good speech
frame is available.  For GSM-FR codec the substitution/muting spec
(GSM 06.11) explicitly defines a silence frame for this purpose,
but no such officially defined fill frames exist for HR or EFR.
Those newer codecs do, however, have homing frames that all decoders
are required to implement, and there is an unofficial silence frame
for GSM-HR, observed as the output from the official encoder fed with
all-zeros input.

Add these 3 constant frames (HR silence, HR homing, EFR homing) to
libosmocodec in order to make them easily usable in any application
that may need them, including applications that cannot have ThemWi
codec libraries as dependencies.

Change-Id: I2c510ac62a0786c137115c45eee7a48b9736265f