gdb: actually disable gdbserver if BR2_PACKAGE_GDB_SERVER is unset

The gdb configure script is given --enable-gdbserver when
BR2_PACKAGE_GDB_SERVER is set, but it is not given --disable-gdbserver
when BR2_PACKAGE_GDB_SERVER is unset.

gdb gdb/configure.ac defaults to enabling gdbserver in "native"
(host=target) cases, which is always the case when buildroot builds a
gdb which runs on the target hardware. The gdbserver will overwrite
BR2_TOOLCHAIN_EXTERNAL_GDB_SERVER_COPY gdbserver, if any.

Fix that by passing --disable-gdbserver when BR2_PACKAGE_GDB_SERVER is
unset.

Signed-off-by: Anssi Hannula <anssi.hannula@bitwise.fi>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>