find_efi_loader is missing bindings for aarch64 artifacts

[Alan Baghumian]

Curtin currently utilizes the find_efi_loader function in the install_grub.py module to determine the correct utility to be used during the GRUB installation stage.

The current version only covers artifacts for x86_64 architecture, hence causing deployment failures (due to GRUB installation errors) on aarch64:

def find_efi_loader(target, bootid):
    efi_path = '/boot/efi/EFI'
    possible_loaders = [
        os.path.join(efi_path, bootid, 'shimx64.efi'),
        os.path.join(efi_path, 'BOOT', 'BOOTX64.EFI'),
        os.path.join(efi_path, bootid, 'grubx64.efi'),
    ]
    for loader in possible_loaders:
        tloader = target_path(target, path=loader)
        if os.path.exists(tloader):
            LOG.debug('find_efi_loader: found %s', loader)
            return loader
    return None

I'll be submitting a PR to fix this issue.