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.

configure.ac

@@ -187,9 +187,13 @@ case "$host_os" in
 esac
 bootdirname=`echo "$bootdirname" | sed "$program_transform_name"`
 AC_SUBST(bootdirname)
+AC_DEFINE_UNQUOTED(GRUB_BOOT_DIR_NAME, "$bootdirname",
+    [Default boot directory name]")
 
 grubdirname=`echo "$PACKAGE" | sed "$program_transform_name"`
 AC_SUBST(grubdirname)
+AC_DEFINE_UNQUOTED(GRUB_DIR_NAME, "$grubdirname",
+    [Default grub directory name])
 
 #
 # Checks for build programs.