efi_loader: rename check_disk_has_default_file function

check_disk_has_default_file() function checks if the
architecture-specific default file exists on the block
device, and fills the default file device path if it exists.

Rename the function name to fill_default_file_path().

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>