efi_loader: move distro_efi_get_fdt_name()

Move distro_efi_get_fdt_name() to a separate C module
and rename it to efi_get_distro_fdt_name().

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>