Avoid conflicts with SDL_CONFIG variable.

Other Android.mk parsed before this one may define SDL_CONFIG
in an unexpected way. We only use this variable as input when
want to implement the --sdl-config option in ./android-configure.sh

Use QEMU_SDL_CONFIG instead to avoid issues.

Change-Id: I89d56284017c696227d7c66ea7d7b0acaa73a9ca
diff --git a/Makefile.common b/Makefile.common
index 5d1aa6e..0b9f954 100644
--- a/Makefile.common
+++ b/Makefile.common
@@ -192,8 +192,9 @@
 
 # Except if we used android-configure.sh --sdl-config=<script>
 #
-ifneq ($(SDL_CONFIG),)
+ifneq ($(QEMU_SDL_CONFIG),)
    BUILD_SDL_FROM_SOURCES := false
+   SDL_CONFIG := $(QEMU_SDL_CONFIG)
 endif
 
 ifneq ($(BUILD_SDL_FROM_SOURCES),true)