Move bootretry code into bootretry.c and clean up

This code is only used by one board, so it seems a shame to clutter up
the readline code with it. Move it into its own file.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/common/Makefile b/common/Makefile
index 81721f9..391a8d6 100644
--- a/common/Makefile
+++ b/common/Makefile
@@ -29,6 +29,11 @@
 obj-y += autoboot.o
 endif
 
+# This option is not just y/n - it can have a numeric value
+ifdef CONFIG_BOOT_RETRY_TIME
+obj-y += bootretry.o
+endif
+
 # boards
 obj-$(CONFIG_SYS_GENERIC_BOARD) += board_f.o
 obj-$(CONFIG_SYS_GENERIC_BOARD) += board_r.o