toolchain: expose BR2_TOOLCHAIN_EXTRA_EXTERNAL_LIBS for all toolchain types

toolchain: expose BR2_TOOLCHAIN_EXTRA_EXTERNAL_LIBS for all toolchain types

This patch extends the "copy extra GCC libraries to target" feature to
also work for internal toolchains. The variable has been renamed to be
BR2_TOOLCHAIN_EXTRA_LIBS and the configuration option moved under the
generic toolchain package. For external toolchains, the step that does
the copy is still in the copy_toolchain_lib_root() helper which copies
from the sysroot to the target.  For the internal toolchain, the host
gcc-final package does a post install hook to copy the libraries from
the toolchain build folders to both the sysroot and target(!static).

Examples where this can be useful is for adding debug libraries to the
target like the GCC libsanitizer (libasan/liblsan/...).

Cc: Markus Mayer <mmayer@broadcom.com>
Signed-off-by: Matthew Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>