build: use target 'debugserver' for Qemu debugging

Use the existing debugserver target also for Qemu debugging. Qemu
should be maintained as one of many emulation/simulations platforms and
emulation should be abstracted in the Makefiles and not tied to Qemu.

qemugdb will still work, it is however being deprecated.

Change-Id: I0cd10fb66debb939b8f7f1304bf2ef4605da6a1d
Signed-off-by: Anas Nashif <anas.nashif@intel.com>