Add a sysctl to report the firmware build number.

Some older firmware versions have issues that can be worked around by
avoiding certain operations.  Add a sysctl dev.aac.#.firmware_build to
make it easy for scripts or userland tools to detect the firmware
version.

sys/dev/aac/aac.c

@@ -291,6 +291,15 @@ aac_attach(struct aac_softc *sc)
 	 */
 	aac_describe_controller(sc);
 
+	/*
+	 * Add sysctls.
+	 */
+	SYSCTL_ADD_INT(device_get_sysctl_ctx(sc->aac_dev),
+	    SYSCTL_CHILDREN(device_get_sysctl_tree(sc->aac_dev)),
+	    OID_AUTO, "firmware_build", CTLFLAG_RD,
+	    &sc->aac_revision.buildNumber, 0,
+	    "firmware build number");
+
 	/*
 	 * Register to probe our containers later.
 	 */