This function packs P7 messages. At least RX_data.indication messages
might be much bigger than the global buffer that is used here. Allocate
the buffer on the stack, and make it bigger. Do not use the global
buffer, it's simply not necessary; increasing the global tx buffer size
might have negative effects elsewhere in the code.

That change might make the function reentrant. The mutex seems to only
guards the codec config. However, leave it to not introduce any
regressions as of now.