[console] Sleep while waiting for user input

Reduce CPU usage while waiting for user input.  This is particularly
important for virtual machines, where CPU is a shared resource.

Reported-by: Alessandro Salvatori <alessandro@embrane.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/getkey.c

 #include <ipxe/process.h>
 #include <ipxe/keys.h>
 #include <ipxe/timer.h>
+#include <ipxe/nap.h>
 
 /** @file
  *
 		step();
 		if ( iskey() )
 			return getchar();
+		cpu_nap();
 	}
 
 	return -1;