debug: do not force installation of gdbserver

The BR2_ENABLE_DEBUG option selects the compilation and installation
of gdbserver on the target. This is a bit restrictive, especially for
external toolchains, which may already contain a gdbserver on the
target.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>