helper/log: add LOG_CUSTOM_LEVEL() macro

Allow logging at a changeable level.

Add an example of usage in ftdi driver.
Log SWD commands with not OK response at debug level (3).
For commands which responded OK use debug io level (4)
not to flood the log.

Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Change-Id: I67a472b293f7ed9ee84cadb7c081803e9eeb1ad0
Reviewed-on: https://review.openocd.org/c/openocd/+/8151
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Tested-by: jenkins

src/helper/log.h

@@ -114,6 +114,15 @@ extern int debug_level;
 				expr); \
 	} while (0)
 
+#define LOG_CUSTOM_LEVEL(level, expr ...) \
+	do { \
+		enum log_levels _level = level; \
+		if (debug_level >= _level) \
+			log_printf_lf(_level, \
+				__FILE__, __LINE__, __func__, \
+				expr); \
+	} while (0)
+
 #define LOG_INFO(expr ...) \
 	log_printf_lf(LOG_LVL_INFO, __FILE__, __LINE__, __func__, expr)