sensors: pull the initialise function out into a header.

Signed-off-by: Michael Hope <mlhx@google.com>

src/main/sensors/initialisation.h

+/*
+ * This file is part of Cleanflight.
+ *
+ * Cleanflight is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * Cleanflight is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with Cleanflight.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#pragma once
+
+bool sensorsAutodetect(sensorAlignmentConfig_t *sensorAlignmentConfig, uint16_t gyroLpf, uint8_t accHardwareToUse, uint8_t magHardwareToUse, int16_t magDeclinationFromConfig);