build: Fix qemugdb target

Even deprecated, this target should still work, the issue was
introduced by commit 2bc9d699.

The problem is that for both of those targets the board's specific
Makefiles should be include'd, it was done only for the non-deprecated
target ('debugserver').

As a note: that is the recommended way of doing a logical OR operation
in Makefiles.

Change-Id: I3ae8f5201c47e65b33a62cea45e25dc2226de489
Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
(cherry picked from commit 370571b5)