Makefile: Match board name with compatible string and install sysfs_attr helper

There is an script that parses /proc/device-tree/compatible and if
matches with the name of the board script runs the test suite. The
Samsung Chromebook Plus reports the following compatibles:

 google,kevin-rev15 google,kevin-rev14 google,kevin-rev13
 google,kevin-rev12 google,kevin-rev11 google,kevin-rev10
 google,kevin-rev9 google,kevin-rev8 google,kevin-rev7
 google,kevin-rev6 google,kevin google,gru rockchip,rk3399

Being google,kevin the more generict this patch renames the board test
script to this name.

The patch also adds the assert_sysfs_attr_present helper script to the
Makefile install target because it is used by the google,kevin test
suite.

Fixes: 1c42025152 ("Makefile: fixes board name for google,kevin")
Fixes: c5d93c41f8 ("boards: Add Samsung Chromebook Plus test script")
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>

Makefile

@@ -8,6 +8,7 @@ HELPERS := assert_device_present \
            assert_driver_present \
 	   assert_mmc_present \
 	   assert_partition_found \
+	   assert_sysfs_attr_present \
 	   bootrr \
 	   ensure_lib_firmware \
 	   rproc-start \
@@ -16,7 +17,7 @@ HELPERS := assert_device_present \
 	   state_check
 
 BOARDS := arrow,apq8096-db820c \
-	  google,rk3399-kevin \
+	  google,kevin \
 	  qcom,apq8016-sbc \
 	  qcom,msm8998-mtp \
 	  qcom,sdm845-mtp \