pcu_l1_if_phy: flexible phy access

At the moment we have three different platform specific PHY interfaces,
which can be selected using a compile time option. Since those three all
are intended to be used on embedded platforms this works fine. However,
in the future we plan to add E1 phy support (PHY/CCU is attached via an
E1 line), which shall not be a compiletime distinction. It might be also
thinkable that we might even add multiple E1 phys in the future, so
multiple phy implementations must be able to co-exist.

This patch adds a generic interface that allows for the creation of PHY
modules that are able to register themselves so that they can be
selected dynamically. The "legacy" PHY implementations are not intended
to co-exist next to other PHYs, so it is possible to register them as
"singleton" modules (only one at a time, via compiletime option). For the
moment only "singleton" modules are supported. The selection mechanism
(pcu_sock, pcu_l1_if.cpp) will be introduced with the first resident PHY
implementation (Ericsson E1 CCU support)

Change-Id: I8692d1bd5d137a17cf596ee2914722f419c9978d
Related: OS#5198
---
M src/Makefile.am
M src/gprs_pcu.h
M src/gprs_rlcmac_sched.cpp
M src/osmo-bts-litecell15/lc15_l1_if.c
M src/osmo-bts-oc2g/oc2g_l1_if.c
M src/osmo-bts-sysmo/sysmo_l1_if.c
M src/osmobts_sock.c
M src/pcu_l1_if.cpp
A src/pcu_l1_if_phy.c
M src/pcu_l1_if_phy.h
M src/pcu_main.cpp
11 files changed, 283 insertions(+), 71 deletions(-)