efi_loader: fix possible buffer overflow

Variable "final" will have SHA512 digest, but currently
the array size is not sufficient. Let's fix it.

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>