efi_loader: Increase number of configuration tables to 16

At present the number of configuration tables is set to 2. By
looking at which tables the Linux EFI stub or iPXE can process,
it looks 16 is a reasonable number.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>