ACPI / tables: table override from built-in initrd

In some scenario, we need to build initrd with kernel in a single image.
This can simplify system deployment process by downloading the whole system
once, such as in IC verification.

This patch adds support to override ACPI tables from built-in initrd.

Signed-off-by: Shunyong Yang <shunyong.yang@hxt-semitech.com>
[ rjw: Minor cleanups ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>