[ncp] add new spinel capability SPINEL_CAP_THREAD_BORDER_ROUTER (#3305)

This capability mirrors `OPENTHREAD_ENABLE_BORDER_ROUTER` feature
informing host that on-mesh prefix or off-mesh routes can be added
on NCP.

src/ncp/ncp_base.cpp

@@ -1798,6 +1798,10 @@ template <> otError NcpBase::HandlePropertyGet<SPINEL_PROP_CAPS>(void)
     SuccessOrExit(error = mEncoder.WriteUintPacked(SPINEL_CAP_THREAD_JOINER));
 #endif
 
+#if OPENTHREAD_ENABLE_BORDER_ROUTER
+    SuccessOrExit(error = mEncoder.WriteUintPacked(SPINEL_CAP_THREAD_BORDER_ROUTER));
+#endif
+
 #if OPENTHREAD_ENABLE_UDP_FORWARD
     SuccessOrExit(error = mEncoder.WriteUintPacked(SPINEL_CAP_THREAD_UDP_FORWARD));
 #endif

src/ncp/spinel.c

@@ -2483,6 +2483,10 @@ const char *spinel_capability_to_cstr(unsigned int capability)
         ret = "THREAD_JOINER";
         break;
 
+    case SPINEL_CAP_THREAD_BORDER_ROUTER:
+        ret = "THREAD_BORDER_ROUTER";
+        break;
+
     case SPINEL_CAP_NEST_LEGACY_INTERFACE:
         ret = "NEST_LEGACY_INTERFACE";
         break;

src/ncp/spinel.h

@@ -763,12 +763,13 @@ enum
     SPINEL_CAP_POSIX_APP               = (SPINEL_CAP_OPENTHREAD__BEGIN + 9),
     SPINEL_CAP_OPENTHREAD__END         = 640,
 
-    SPINEL_CAP_THREAD__BEGIN       = 1024,
-    SPINEL_CAP_THREAD_COMMISSIONER = (SPINEL_CAP_THREAD__BEGIN + 0),
-    SPINEL_CAP_THREAD_TMF_PROXY    = (SPINEL_CAP_THREAD__BEGIN + 1),
-    SPINEL_CAP_THREAD_UDP_FORWARD  = (SPINEL_CAP_THREAD__BEGIN + 2),
-    SPINEL_CAP_THREAD_JOINER       = (SPINEL_CAP_THREAD__BEGIN + 3),
-    SPINEL_CAP_THREAD__END         = 1152,
+    SPINEL_CAP_THREAD__BEGIN        = 1024,
+    SPINEL_CAP_THREAD_COMMISSIONER  = (SPINEL_CAP_THREAD__BEGIN + 0),
+    SPINEL_CAP_THREAD_TMF_PROXY     = (SPINEL_CAP_THREAD__BEGIN + 1),
+    SPINEL_CAP_THREAD_UDP_FORWARD   = (SPINEL_CAP_THREAD__BEGIN + 2),
+    SPINEL_CAP_THREAD_JOINER        = (SPINEL_CAP_THREAD__BEGIN + 3),
+    SPINEL_CAP_THREAD_BORDER_ROUTER = (SPINEL_CAP_THREAD__BEGIN + 4),
+    SPINEL_CAP_THREAD__END          = 1152,
 
     SPINEL_CAP_NEST__BEGIN           = 15296,
     SPINEL_CAP_NEST_LEGACY_INTERFACE = (SPINEL_CAP_NEST__BEGIN + 0),