libtirpc: prevent selection when ADI Blackfin external toolchains are used

libtirpc fails to build with the Blackfin external toolchains provided
by Analog Devices, because their uClibc configuration doesn't include
AI_ADDRCONFIG. In order to prevent such build failures to happen, we
simply prevent libtirpc from being selected when such toolchains are
used. This is not a big problem, since they provide native RPC
support.

Also, since they provide native RPC, we don't have to propagate this
new dependency to the reverse dependencies of libtirpc, because they
all use native RPC when available. The exception to this rule is the
rpcbind package, which can only use the libtirpc implementation of
RPCs, and not the native one of C libraries. Therefore, the dependency
is propagated to the rpcbind package.

Fixes:

  http://autobuild.buildroot.org/results/58b/58b16449065c16afce11ba120db56839efb2b1ea/build-end.log



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>