eficonfig: add "Edit Boot Option" menu entry

This commit adds the menu entry to edit the existing
BOOT#### variable contents.
User selects the item from the boot option list, then
user can edit the description, file path and optional_data.

Note that automatically generated boot option entry by bootmenu
to support the removable media device is filtered out and user
can not edit the automatically generated entry.

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
diff --git a/include/efi_config.h b/include/efi_config.h
index ea5bc4e..dddffe0 100644
--- a/include/efi_config.h
+++ b/include/efi_config.h
@@ -11,6 +11,7 @@
 #include <efi_loader.h>
 
 #define EFICONFIG_ENTRY_NUM_MAX 99
+#define EFICONFIG_VOLUME_PATH_MAX 512
 #define EFICONFIG_FILE_PATH_MAX 512
 #define EFICONFIG_FILE_PATH_BUF_SIZE (EFICONFIG_FILE_PATH_MAX * sizeof(u16))