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>