external-toolchain: hardcode the destination directory for a library

Until now, the function copy_toolchain_lib_root was copying a given
library to the target filesystem by assuming that it should be at the
same place it was in the toolchain sysroot.

However, with Buildroot hiding libstdc++ in
/usr/<target-name>/lib(64), this isn't correct, and it is probably
safer not to rely on the toolchain organization anyway.

Therefore :

 * Instead of having a single EXTERNAL_LIBS variable, we now have
   LIB_EXTERNAL_LIBS and USR_LIB_EXTERNAL_LIBS, which respectively
   list the libraries that should be copied to /lib and /usr/lib. As
   of today, only libstdc++ is part of the second list.

 * The copy_toolchain_lib_root takes another argument, which is the
   destination directory of the library, relative to $(TARGET_DIR)

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