Add a --locale-directory option to grub-install and related tools.

Add a --locale-directory option to grub-install and related tools.

* include/grub/util/install.h (GRUB_INSTALL_OPTIONS): Add
--locale-directory option.
(enum grub_install_options): Add
GRUB_INSTALL_OPTIONS_LOCALE_DIRECTORY.
* util/grub-install-common.c (grub_install_help_filter): Handle
GRUB_INSTALL_OPTIONS_LOCALE_DIRECTORY.
(grub_install_parse): Likewise.
(get_localedir): New function to check for a user-provided option
before trying grub_util_get_localedir.
(copy_locales): Use get_localedir rather than
grub_util_get_localedir.  Handle differing locale directory layouts.
(grub_install_copy_files): Likewise.