trxcon: rework trxcon_fsm, move into a separate file

The original trxcon_fsm I wrote back in 2017 [1] was more like
a boolean flag, as there were only two states: IDLE and MANAGED.
Not surprising, given that until recently handling of multiple
L1CTL connections was not supported.  Now that we have this
implemented, lifetime of a trxcon_fsm instance is limited by
lifetime of a L1CTL connection, what renders the FSM useless.

This change removes the old 'boolean' trxcon_fsm and introduces
the new one, which will allows us to abstract the L1CTL interface
from the TRXC/TRXD interfaces, as well as the scheduler.  The new
FSM will also simplify development of the RLC/MAC layer for GPRS.

Change-Id: Ifaf63ead9dd180181358e771367b2a686ba159ca
Related: [1] I7ee6fc891abe5f775f5b7ebbf093181a97950dea
Related: OS#5599
---
M src/host/trxcon/include/osmocom/bb/l1sched/l1sched.h
M src/host/trxcon/include/osmocom/bb/trxcon/l1ctl.h
M src/host/trxcon/include/osmocom/bb/trxcon/trxcon.h
M src/host/trxcon/src/Makefile.am
M src/host/trxcon/src/l1ctl.c
M src/host/trxcon/src/trx_if.c
M src/host/trxcon/src/trxcon.c
A src/host/trxcon/src/trxcon_fsm.c
8 files changed, 787 insertions(+), 411 deletions(-)