hps_accel/Makefile: use gen1 gateware
hps_accel/Makefile: use gen1 gateware

gen2 gateware does not yet pass golden tests, so set default to
gen2.

Signed-off-by: Alan Green <97e77871b5fcc4941bebceb4338c97c1fd149f8c@google.com>

google/CFU-Playground,google/CFU-Playground,google/CFU-Playground,google/CFU-Playground