Trace: Change tr_debug color from gray to bright blue

The gray-on-black color code used for debug level print-out in
mbed_trace is hard to read. Bright-blue-on-black increases the
brightness of the text without over shadowing the info level
default (white-on-black), thus making it easier to read while
maintaining the original intention.

For original color set MBED_TRACE_COLOR_THEME to 1.

platform/mbed-trace/mbed_lib.json

@@ -19,6 +19,12 @@
             "value": "malloc",
             "macro_name": "MEM_ALLOC"
         },
+        "color-theme": {
+            "help": "Set color theme. 0 for readable, 1 for unobtrusive.",
+            "options": [0, 1],
+            "macro_name": "MBED_TRACE_COLOR_THEME",
+            "value": 0
+        },
         "deallocator": {
             "value": "free",
             "macro_name": "MEM_FREE"

platform/mbed-trace/source/mbed_trace.c

@@ -49,10 +49,17 @@
 #endif
 #endif /* YOTTA_CFG_MEMLIB */
 
+#if defined(MBED_TRACE_COLOR_THEME) && (MBED_TRACE_COLOR_THEME == 1)
 #define VT100_COLOR_ERROR "\x1b[31m"
 #define VT100_COLOR_WARN  "\x1b[33m"
 #define VT100_COLOR_INFO  "\x1b[39m"
 #define VT100_COLOR_DEBUG "\x1b[90m"
+#else
+#define VT100_COLOR_ERROR "\x1b[31m"
+#define VT100_COLOR_WARN  "\x1b[33m"
+#define VT100_COLOR_INFO  "\x1b[39m"
+#define VT100_COLOR_DEBUG "\x1b[94m"
+#endif
 
 /** default max trace line size in bytes */
 #ifdef MBED_TRACE_LINE_LENGTH