examples: detect default build directory

examples: detect default build directory

Most examples have in their makefiles a default RTE_TARGET directory to be
used in case RTE_TARGET is not set. Rather than just using a hard-coded
default, we can instead detect what the build directory is relative to
RTE_SDK directory.

This fixes a potential issue for anyone who continues to build using
"make install T=x86_64-native-linuxapp-gcc" and skips setting RTE_TARGET
explicitly, instead relying on the fact that they were building in a
directory which corresponded to the example default path - which was
changed to "x86_64-native-linux-gcc" by commit 218c4e68c1d9 ("mk: use
linux and freebsd in config names").

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Ferruh Yigit <ferruh.yigit@intel.com>