Move autoboot code to autoboot.c

The autoboot code is complex and long. It deserves its own file with
a simple interface from main.c.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/common/Makefile b/common/Makefile
index 7998325..ae79865 100644
--- a/common/Makefile
+++ b/common/Makefile
@@ -23,6 +23,11 @@
 obj-y += xyzModem.o
 obj-y += cmd_disk.o
 
+# This option is not just y/n - it can have a numeric value
+ifdef CONFIG_BOOTDELAY
+obj-y += autoboot.o
+endif
+
 # boards
 obj-$(CONFIG_SYS_GENERIC_BOARD) += board_f.o
 obj-$(CONFIG_SYS_GENERIC_BOARD) += board_r.o