BSC: introduce new "sysmobts" BTS model

so far, osmo-bts/sysmobts used to be entered as "sysmobts" type in the
configuration file.  However, there are some differences in the
protocol/behaviour and we should reflect that by a new BTS plugin (with
lots of code reuse from the nanobts driver).
diff --git a/openbsc/src/libbsc/Makefile.am b/openbsc/src/libbsc/Makefile.am
index 6f170e0..7cb1e6e 100644
--- a/openbsc/src/libbsc/Makefile.am
+++ b/openbsc/src/libbsc/Makefile.am
@@ -14,6 +14,7 @@
 			bts_nokia_site.c \
 			bts_hsl_femtocell.c \
 			bts_unknown.c \
+			bts_sysmobts.c \
 			chan_alloc.c \
 			gsm_subscriber_base.c \
 			handover_decision.c handover_logic.c meas_rep.c \