Increase warning level & Fix all warnings

As discussed in !357 (merged), we should increase the number of enabled compiler warnings to catch a lot more silent bugs. Increase the meson-global warning level to 2, but excluding the unused-parameter and old-style-declaration warnings. Additionally, disable warnings for libraries as we cannot fix that code anyway.

Fixes #192 (closed)

Cc: @fuchsi

epicardium/modules/bhi.c

@@ -227,7 +227,7 @@ out_free_i2c:
 
 void epic_bhi160_disable_all_sensors()
 {
-	for (int i = 0; i < sizeof(bhi160_sensor_active); i++) {
+	for (size_t i = 0; i < sizeof(bhi160_sensor_active); i++) {
 		if (bhi160_sensor_active[i]) {
 			epic_bhi160_disable_sensor(i);
 		}