configure: Let the TARGET_GPROF var use the regular 'y' for Yes

All other variables are set using 'y', which is what the rules.mak
functions expect to parse.
Signed-off-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190103150951.17592-2-philmd@redhat.com>
Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>

configure

@@ -7490,7 +7490,7 @@ alpha)
 esac
 
 if test "$gprof" = "yes" ; then
-  echo "TARGET_GPROF=yes" >> $config_target_mak
+  echo "TARGET_GPROF=y" >> $config_target_mak
   if test "$target_linux_user" = "yes" ; then
     cflags="-p $cflags"
     ldflags="-p $ldflags"