[ipaccess-config] fix bugs in generating the PHYSICAL CONFIG attribute

... while asking the BTS to perform tests for us.  The length of the
ARFCN whitelist is the actual length in bytes, not the number of 16bit
ARFCN numbers.

Also, implement a limit, either by rxlevel or by number of ARFCN
that should end up in the whitelist.
diff --git a/openbsc/src/abis_nm.c b/openbsc/src/abis_nm.c
index 2d8f687..aaf99ee 100644
--- a/openbsc/src/abis_nm.c
+++ b/openbsc/src/abis_nm.c
@@ -1058,7 +1058,8 @@
 	if (oh->placement != ABIS_OM_PLACEMENT_ONLY) {
 		LOGP(DNM, LOGL_ERROR, "ABIS OML placement 0x%x not supported\n",
 			oh->placement);
-		return -EINVAL;
+		if (oh->placement != ABIS_OM_PLACEMENT_FIRST)
+			return -EINVAL;
 	}
 	if (oh->sequence != 0) {
 		LOGP(DNM, LOGL_ERROR, "ABIS OML sequence 0x%x != 0x00\n",