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 <thomas.monjalon@6wind.com>