efi: x86: Correct the condition for installing ACPI tables

It is not always the case that U-Boot builds the ACPI tables itself. For
example, when booting from coreboot, the ACPI tables are built by
coreboot.

Correct the Makefile condition so that U-Boot can pass on tables built
by a previous firmware stage.

Tidy up the installation-condition code while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>