toolchain: Allow external toolchains to specify libthread-db

We need to let external toolchains be able to specify the path and
specification file to the libthread-db POSIX thread debugging shared
libraries.

This fixes GDB not being able to be installed because it is depending on
libthread-db:

Collected errors:
 * satisfy_dependencies_for: Cannot satisfy the following dependencies
 * for gdb:
 *      libthread-db *
 * opkg_install_cmd: Cannot install package gdb.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>