configure: Drop unneeded TARGET_CFLAGS expansion

Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Leif Lindholm <leif@nuviainc.com>

configure.ac

@@ -77,7 +77,7 @@ grub_TRANSFORM([grub-file])
 
 # Optimization flag.  Allow user to override.
 if test "x$TARGET_CFLAGS" = x; then
-  TARGET_CFLAGS="$TARGET_CFLAGS -Os"
+  TARGET_CFLAGS=-Os
 fi
 
 # Default HOST_CPPFLAGS