toolchain/external: better report RPC error for custom toolchains

Currently, we instruct users to enable/disable BR2_TOOLCHAIN_HAS_NATIVE_RPC
but that is a blind option. The only option users can set/unset is
BR2_TOOLCHAIN_EXTERNAL_INET_RPC.

Use that in the error message.

Notes: the only way for this message to appear is for a custom external
toolchain, either downloaded or pre-installed, so even though we check
the validity of the toolchain with BR2_TOOLCHAIN_HAS_NATIVE_RPC, we do
report on BR2_TOOLCHAIN_EXTERNAL_INET_RPC.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

toolchain/helpers.mk

@@ -197,11 +197,11 @@ check_glibc_feature = \
 check_glibc_rpc_feature = \
 	IS_IN_LIBC=`test -f $(1)/usr/include/rpc/rpc.h && echo y` ; \
 	if [ "$(BR2_TOOLCHAIN_HAS_NATIVE_RPC)" != "y" -a "$${IS_IN_LIBC}" = "y" ] ; then \
-		echo "RPC support available in C library, please enable BR2_TOOLCHAIN_HAS_NATIVE_RPC" ; \
+		echo "RPC support available in C library, please enable BR2_TOOLCHAIN_EXTERNAL_INET_RPC" ; \
 		exit 1 ; \
 	fi ; \
 	if [ "$(BR2_TOOLCHAIN_HAS_NATIVE_RPC)" = "y" -a "$${IS_IN_LIBC}" != "y" ] ; then \
-		echo "RPC support not available in C library, please disable BR2_TOOLCHAIN_HAS_NATIVE_RPC" ; \
+		echo "RPC support not available in C library, please disable BR2_TOOLCHAIN_EXTERNAL_INET_RPC" ; \
 		exit 1 ; \
 	fi