platform: relax rom scanning alignment for ahci platforms

platform: relax rom scanning alignment for ahci platforms

The PCI-3.0 Firmware specification allows for option-roms to have
512-byte alignment rather than 2048-byte.  As there does not appear to
be a reliable method to detect a PCI-3.0 compliant BIOS from userspace
we allow the imsm platform detection code to presume that a system
modern enough to have an Intel AHCI controller does not have
dangerous/legacy ISA regions in the option-ROM memory space.

An environment variable to disable this behaviour, IMSM_SAFE_OROM_SCAN,
is added in case this presumption is ever proven wrong.

Reported-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>