[romprefix] Fully clear the "Press B to boot..." message when INT19 is used

src/arch/i386/prefix/romprefix.S

 	.asciz	"\nPress Ctrl-B to configure gPXE..."
 	.size	init_message_prompt, . - init_message_prompt
 clear_message:
-	.asciz	"\r                                     \n\n"
+	.asciz	"\r                                          \n\n"
 	.size	clear_message, . - clear_message
 
 /* ROM image location