toolchain-external: align library locations in target and staging dir

toolchain-external: align library locations in target and staging dir

The toolchain-external logic is roughly:
- populate the staging dir by rsyncing the entire ${ARCH_LIB_DIR} and
  usr/${ARCH_LIB_DIR} from sysroot.
- populate the target dir by explictly copying some libraries from sysroot
  into target/lib and some other libraries in target/usr/lib, the split
  being hardcoded into buildroot regardless of the location in the sysroot.

This means that a library libfoo could be located in:
  staging/lib/libfoo.so
  target/usr/lib/libfoo.so

When debugging an application that links against this library, gdb will
fruitlessly search for 'usr/lib/libfoo.so' in staging, and then suggest to
use 'set solib-search-path' which is a hack, really.

To solve the problem, we need to make sure that libraries from the toolchain
are installed in the same relative location in staging and target.
Achieve this by:
- replacing the convoluted search for libraries using for+find in sysroot
  with a simple find in staging.
- determining DESTDIR for each library individually based on the location in
  staging.
- treating LIB_EXTERNAL_LIBS and USR_LIB_EXTERNAL_LIBS equivalently

These changes also allow for the removal of most arguments to
copy_toolchain_lib_root in the method itself and their callers.

Test procedure:
- set configuration for a given toolchain
- make clean toolchain
- find output/target | sort > /tmp/out-before
- apply patch
- make clean toolchain
- find output/target | sort > /tmp/out-after
- diff -u /tmp/out-before /tmp/out-after

The only changes should be some libraries moving from lib to usr/lib or vice
versa. Notable examples being libstdc++ and libatomic.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
[Thomas:
 - use -L instead of -follow in the find invocation, as suggested by
   Arnout.
 - move the BR2_STATIC_LIBS condition as a make condition rather than
   a shell condition, as suggested by Arnout.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>