Keep boot and grub directory names in sync with utils scripts

* configure.ac: Define GRUB_BOOT_DIR_NAME and GRUB_DIR_NAME macros.
    * config.h.in: Add previous macros.
    * include/grub/emu/misc.h (DEFAULT_DIRECTORY): Use previous macros.
    * util/grub-install.in: Use $bootdir and $grubdir variables.

config.h.in

@@ -24,6 +24,10 @@
 #define PACKAGE_NAME "@PACKAGE_NAME@"
 /* Define to the address where bug reports for this package should be sent. */
 #define PACKAGE_BUGREPORT "@PACKAGE_BUGREPORT@"
+/* Default boot directory name" */
+#define GRUB_BOOT_DIR_NAME "@bootdirname@"
+/* Default grub directory name */
+#define GRUB_DIR_NAME "@grubdirname@"
 /* Define to 1 if GCC generates calls to __enable_execute_stack().  */
 #define NEED_ENABLE_EXECUTE_STACK @NEED_ENABLE_EXECUTE_STACK@
 /* Define to 1 if GCC generates calls to __register_frame_info().  */