GPRS: change 'gprs enabled <0-1>' to 'gprs mode (none|gprs|egprs)'

This causes some config file breakage but sounds like a much cleaner
approach than to have two separate config variables for this.
diff --git a/openbsc/src/system_information.c b/openbsc/src/system_information.c
index de0e287..8072ba2 100644
--- a/openbsc/src/system_information.c
+++ b/openbsc/src/system_information.c
@@ -458,7 +458,15 @@
 
 int gsm_generate_si(u_int8_t *output, struct gsm_bts *bts, int type)
 {
-	si_info.gprs_ind.present = bts->gprs.enabled;
+	switch (bts->gprs.mode) {
+	case BTS_GPRS_EGPRS:
+	case BTS_GPRS_GPRS:
+		si_info.gprs_ind.present = 1;
+		break;
+	case BTS_GPRS_NONE:
+		si_info.gprs_ind.present = 0;
+		break;
+	}
 
 	switch (type) {
 	case RSL_SYSTEM_INFO_1: