[init] Remove concept of "shutdown exit flags"

Remove the concept of shutdown exit flags, and replace it with a
counter used to keep track of exposed interfaces that require devices
to remain active.

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

src/arch/i386/image/bzimage.c

@@ -477,7 +477,7 @@ static int bzimage_exec ( struct image *image ) {
 	bzimage_update_header ( image, &bzimg, bzimg.rm_kernel );
 
 	/* Prepare for exiting */
-	shutdown ( SHUTDOWN_BOOT );
+	shutdown_boot();
 
 	DBGC ( image, "bzImage %p jumping to RM kernel at %04x:0000 "
 	       "(stack %04x:%04zx)\n", image, ( bzimg.rm_kernel_seg + 0x20 ),

src/arch/i386/image/elfboot.c

@@ -48,7 +48,7 @@ static int elfboot_exec ( struct image *image ) {
 	/* An ELF image has no callback interface, so we need to shut
 	 * down before invoking it.
 	 */
-	shutdown ( SHUTDOWN_BOOT );
+	shutdown_boot();
 
 	/* Jump to OS with flat physical addressing */
 	DBGC ( image, "ELF %p starting execution at %lx\n", image, entry );

src/arch/i386/image/multiboot.c

@@ -278,7 +278,7 @@ static int multiboot_exec ( struct image *image ) {
 	/* Multiboot images may not return and have no callback
 	 * interface, so shut everything down prior to booting the OS.
 	 */
-	shutdown ( SHUTDOWN_BOOT );
+	shutdown_boot();
 
 	/* Build memory map after unhiding bootloader memory regions as part of
 	 * shutting everything down.

src/arch/i386/image/nbi.c

@@ -406,7 +406,7 @@ static int nbi_exec ( struct image *image ) {
 	/* Shut down now if NBI image will not return */
 	may_return = NBI_PROGRAM_RETURNS ( imgheader.flags );
 	if ( ! may_return )
-		shutdown ( SHUTDOWN_BOOT );
+		shutdown_boot();
 
 	/* Execute NBI image */
 	if ( NBI_LINEAR_EXEC_ADDR ( imgheader.flags ) ) {