Enable hwmon and sensor drivers

hwmon core, nct7904, lm75, and at24.
diff --git a/meta-ast2400/recipes-kernel/linux/linux-obmc/hwmon.cfg b/meta-ast2400/recipes-kernel/linux/linux-obmc/hwmon.cfg
new file mode 100644
index 0000000..053fea3
--- /dev/null
+++ b/meta-ast2400/recipes-kernel/linux/linux-obmc/hwmon.cfg
@@ -0,0 +1,4 @@
+CONFIG_HWMON=y
+CONFIG_SENSORS_NCT7904=y
+CONFIG_SENSORS_LM75=y
+CONFIG_EEPROM_AT24=y