gdb: install gdbserver into the SDK

In order for a Buildroot toolchain to be usable as an external
toolchain, we should copy the target gdbserver somewhere into
$(HOST_DIR). In order to match what Crosstool-NG does (and therefore
to match something the external toolchain logic already understands),
we install gdbserver in
$(HOST_DIR)/usr/<tuple>/debug-root/usr/bin/gdbserver.

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