external-toolchain-csl: handle packaging of gdbserver based on PREFERRED_PROVIDER

external-toolchain-csl: handle packaging of gdbserver based on PREFERRED_PROVIDER

Allow packaging independent copy of specific version of gdbserver with
external-toolchain-csl by setting PREFERRED_PROVIDER. E.g. for GPLv2
gdbserver, add these lines to your distro/local.conf:

PREFERRED_PROVIDER_gdbserver = "gdbserver"
PREFERRED_VERSION_gdbserver = "6.6"

Thanks to Ben Gardiner for providing valuable feedback.

Signed-off-by: Denys Dmytriyenko <denys@ti.com>