package/belr: fix BR2_SHARED_STATIC_LIBS build

Fix the following build failure with BR2_SHARED_STATIC_LIBS:

CMake Error at src/CMakeLists.txt:56 (add_library):
  add_library cannot create target "belr" because another target with the
  same name already exists.  The existing target is a static library created
  in source directory

Fixes:
 - http://autobuild.buildroot.org/results/d1ef96c8f370b5a522985c37f1681dd10bbc15bb

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[Peter: simplify/add comment explaining why]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
(cherry picked from commit c9bee083e0baea16eb91af117033d0ff3fd290b1)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/belr/belr.mk

@@ -17,9 +17,8 @@ BELR_CONF_OPTS = \
 
 ifeq ($(BR2_STATIC_LIBS),y)
 BELR_CONF_OPTS += -DENABLE_SHARED=OFF -DENABLE_STATIC=ON
-else ifeq ($(BR2_SHARED_STATIC_LIBS),y)
-BELR_CONF_OPTS += -DENABLE_SHARED=ON -DENABLE_STATIC=ON
-else ifeq ($(BR2_SHARED_LIBS),y)
+else
+# cannot build static and shared together
 BELR_CONF_OPTS += -DENABLE_SHARED=ON -DENABLE_STATIC=OFF
 endif