mk: fix external library link
mk: fix external library link

When building an external library with rte.extlib.mk, the internal
libraries were not found because the linker search path was the
external library install directory (RTE_OUTPUT/lib).
It is fixed by searching in the internal library install directory
(RTE_SDK_BIN/lib).
When building an internal library, RTE_SDK_BIN = RTE_OUTPUT.

Fixes: c6417ce61f83 ("mk: add build-time library directory to linker path")

Signed-off-by: Thomas Monjalon <ee2fa8b2681c737f775687e475dcbf2028a412f1@6wind.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk