[debug] Allow debug colourisation to be disabled

Some BIOS console redirection capabilities do not work well with the
colourised debug messages used by iPXE.  We already allow the range of
colours to be controlled via the DBGCOL=... build parameter.  Extend
this syntax to allow DBGCOL=0 to be used to mean "disable colours".

Requested-by: Wissam Shoukair <wissams@mellanox.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/Makefile.housekeeping

 VERYCLEANUP += $(DBGCOL_LIST)
 
 DBGCOL_COLOURS := $(subst -, ,$(DBGCOL))
-DBGCOL_MIN := $(word 1,$(DBGCOL_COLOURS))
-DBGCOL_MAX := $(word 2,$(DBGCOL_COLOURS))
+DBGCOL_MIN := $(firstword $(DBGCOL_COLOURS))
+DBGCOL_MAX := $(lastword $(DBGCOL_COLOURS))
 
 debug_DEPS += $(DBGCOL_LIST)
 

src/core/debug.c

  * @v stream		Message stream ID
  */
 void dbg_autocolourise ( unsigned long stream ) {
-	dbg_printf ( "\033[%dm",
-		     ( stream ? ( DBGCOL_MIN + dbg_autocolour ( stream ) ) :0));
+
+	if ( DBGCOL_MIN ) {
+		dbg_printf ( "\033[%dm",
+			     ( stream ?
+			       ( DBGCOL_MIN + dbg_autocolour ( stream ) ) : 0));
+	}
 }
 
 /**
  *
  */
 void dbg_decolourise ( void ) {
-	dbg_printf ( "\033[0m" );
+
+	if ( DBGCOL_MIN )
+		dbg_printf ( "\033[0m" );
 }