[cmdline] Added configurable shell banner timeout

This change allows the time for which shell banners are displayed to
be configured in the config.h file.  The ability to access the shell
can also be effectively disabled by setting this timeout to zero.

src/config.h

  */
 #define TIMER_BIOS		/* 18Hz BIOS timer */
 #define TIMER_RDTSC		/* CPU TimeStamp Counter timer */
+#define BANNER_TIMEOUT	20	/* Tenths of a second for which the shell
+				   banner should appear */
 
 /* @END general.h */
 

src/hci/shell_banner.c

 
 #include <stdio.h>
 #include <console.h>
-#include <gpxe/timer.h>
+#include <unistd.h>
+#include <config/general.h>
 #include <gpxe/shell_banner.h>
 
 /** @file
  *
  */
 
-#define BANNER_TIMEOUT ( 2 * TICKS_PER_SEC )
-
 /**
  * Print shell banner and prompt for shell entry
  *
  * @ret	enter_shell		User wants to enter shell
  */
 int shell_banner ( void ) {
-	unsigned long timeout = ( currticks() + BANNER_TIMEOUT );
+	int wait_count = 0;
 	int enter_shell = 0;
 	int key;
 
 	printf ( "\nPress Ctrl-B for the gPXE command line..." );
 
 	/* Wait for key */
-	while ( currticks() < timeout ) {
+	while ( wait_count < BANNER_TIMEOUT ) {
 		if ( iskey() ) {
 			key = getchar();
 			if ( key == 0x02 /* Ctrl-B */ )
 				enter_shell = 1;
 			break;
 		}
+		mdelay(100);
+		wait_count++;
 	}
 
 	/* Clear the "Press Ctrl-B" line */