[U-Boot] efi_loader: add static to local functions

[Alexander Graf]

> These are locally used in lib/efi_loader/efi_boottime.c
> 
> Signed-off-by: Masahiro Yamada <yamada.masahiro at socionext.com>
> Reviewed-by: Bin Meng <bmeng.cn at gmail.com>

Thanks, applied to efi-next

Alex