[console] Add concept of a "magic" colour

The magic basic colour can be remapped at runtime from COLOR_NORMAL_BG
(usually blue) to COLOR_DEFAULT (which will be transparent as a
background colour on the framebuffer console).

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/ansicoldef.c

  * @v basic		Basic colour
  * @ret ansicol		ANSI colour definition
  *
- * Colours default to being just a basic colour.
+ * Colours default to being just a basic colour.  If the colour
+ * matches the normal UI text background colour, then its basic colour
+ * value is set to @c ANSICOL_MAGIC.
  */
-#define ANSICOL_DEFAULT( basic ) ANSICOL_DEFINE ( (basic), ANSICOL_NO_RGB )
+#define ANSICOL_DEFAULT( basic )					\
+	ANSICOL_DEFINE ( ( ( (basic) == COLOR_NORMAL_BG ) ?		\
+			   ANSICOL_MAGIC : (basic) ),			\
+			 ANSICOL_NO_RGB )
 
 /** ANSI colour definitions */
 static uint32_t ansicols[] = {
 	[COLOR_WHITE]	= ANSICOL_DEFAULT ( COLOR_WHITE ),
 };
 
+/** Magic basic colour */
+static uint8_t ansicol_magic = COLOR_NORMAL_BG;
+
 /**
  * Define ANSI colour
  *
 		ansicol = ANSICOL_DEFINE ( COLOUR_DEFAULT, ANSICOL_NO_RGB );
 	}
 
-	/* If basic colour is out of range, use the default colour */
+	/* If basic colour is out of range, use the magic colour */
 	basic = ANSICOL_BASIC ( ansicol );
 	if ( basic >= 10 )
-		basic = COLOR_DEFAULT;
+		basic = ansicol_magic;
 
 	/* Set basic colour first */
 	printf ( CSI "%c%dm", which, basic );
 			 ANSICOL_GREEN ( ansicol ), ANSICOL_BLUE ( ansicol ) );
 	}
 }
+
+/**
+ * Reset magic colour
+ *
+ */
+void ansicol_reset_magic ( void ) {
+
+	/* Set to the compile-time default background colour */
+	ansicol_magic = COLOR_NORMAL_BG;
+}
+
+/**
+ * Set magic colour to transparent
+ *
+ */
+void ansicol_set_magic_transparent ( void ) {
+
+	/* Set to the console default colour (which will give a
+	 * transparent background on the framebuffer console).
+	 */
+	ansicol_magic = COLOR_DEFAULT;
+}

src/include/ipxe/ansicol.h

 #define COLOUR_DEFAULT 9
 #define COLOR_DEFAULT COLOUR_DEFAULT
 
+/** Magic colour
+ *
+ * The magic basic colour is automatically remapped to the colour
+ * stored in @c ansicol_magic.  This is used to allow the UI
+ * background to automatically become transparent when a background
+ * picture is used.
+ */
+#define ANSICOL_MAGIC 15
+
 /** RGB value for "not defined" */
 #define ANSICOL_NO_RGB 0x01000000
 
 /* ansicoldef.c */
 extern int ansicol_define ( unsigned int colour, unsigned int ansi,
 			    uint32_t rgb );
+extern void ansicol_reset_magic ( void );
+extern void ansicol_set_magic_transparent ( void );
 
 /* Function provided by ansicol.c but overridden by ansicoldef.c, if present */
 extern void ansicol_set ( unsigned int colour, unsigned int which );