libbladeRF: Added BLADERF_LOG_LEVEL environment variable

The added BLADERF_LOG_LEVEL environment variable allows the
default libbladeRF log-level to be overridden by the environment.

It may be set to one of the following strings:
 - "critical"
 - "error"
 - "warning"
 - "info"
 - "debug"
 - "verbose"

Bear in mind that applications (such as bladeRF-cli) may control and
change the log level via bladerf_log_set_verbosity(),

host/libraries/libbladeRF/src/init_fini.c

@@ -46,6 +46,7 @@
 #else
 #   define DEF_LOG_LEVEL BLADERF_LOG_LEVEL_INFO
 #endif
+#define ENV_LOG_LEVEL "BLADERF_LOG_LEVEL"
 
 /* Module initializers/deinitializers. When used as library (who don't have
  * a natural entry/exit function) these are used to initialize
@@ -59,22 +60,41 @@
  * the mechanism works.
  */
 
+static int get_loglevel(void) {
+    int log_level = DEF_LOG_LEVEL;
+
+    if ((getenv(ENV_LOG_LEVEL) != NULL)
+        && (strlen(getenv(ENV_LOG_LEVEL)) > 0)) {
+
+        bool valid_value;
+        log_level = str2loglevel(getenv(ENV_LOG_LEVEL), &valid_value);
+
+        if (!valid_value) {
+            log_level = DEF_LOG_LEVEL;
+        }
+    }
+    return log_level;
+}
 
 void __init __bladerf_init(void)
 {
+    int log_level = get_loglevel();
+
 #if !defined(WIN32) && !defined(__CYGWIN__) && defined(LOG_SYSLOG_ENABLED)
     openlog("bladeRF",
             LOG_CONS | LOG_NDELAY | LOG_NOWAIT | LOG_PERROR | LOG_PID,
             LOG_USER);
 #endif
 
-    bladerf_log_set_verbosity(DEF_LOG_LEVEL);
+    bladerf_log_set_verbosity(log_level);
     log_debug("libbladeRF %s: initializing\n", LIBBLADERF_VERSION);
 }
 
 void __fini __bladerf_fini(void)
 {
-    bladerf_log_set_verbosity(DEF_LOG_LEVEL);
+    int log_level = get_loglevel();
+
+    bladerf_log_set_verbosity(log_level);
     log_debug("libbladeRF %s: deinitializing\n", LIBBLADERF_VERSION);
     fflush(NULL);
 #if !defined(WIN32) && !defined(__CYGWIN__) && defined(LOG_SYSLOG_ENABLED)