efi_loader: Use directly version_string variable

Macro U_BOOT_VERSION_STRING is already stored in variable version_string.
So use directly this variable instead of storing U_BOOT_VERSION_STRING into
temporary variable.

Signed-off-by: Pali Rohár <pali@kernel.org>
[trini: This area was reworked since posted, what is here is now really
        inspired by the previous version, so drop Ilias' Acked-by]
Signed-off-by: Tom Rini <trini@konsulko.com>