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/hardware.c

@@ -95,7 +95,7 @@ int hardware_early_init(void)
 		;
 
 	/* If we don't have a valid time yet, set it to 2019-01-01 */
-	if (RTC_GetSecond() < 1546300800UL) {
+	if (RTC_GetSecond() < 1546300800L) {
 		epic_rtc_set_milliseconds(1546300800UL * 1000);
 	}