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>
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