[acpi] Allow for platforms that provide ACPI tables individually

[acpi] Allow for platforms that provide ACPI tables individually

The ACPI API currently expects platforms to provide access to a single
contiguous ACPI table.  Some platforms (e.g. Linux userspace) do not
provide a convenient way to obtain the entire ACPI table, but do
provide access to individual tables.

All iPXE consumers of the ACPI API require access only to individual
tables.

Redefine the internal API to make acpi_find() an API method, with all
existing implementations delegating to the current RSDT-based
implementation.

Signed-off-by: Michael Brown <mcb30@ipxe.org>