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: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190103150951.17592-2-philmd@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>