feature flag change notifications

["dann frazier" <dann.frazier@canonical.com>]

Hi,
  I maintain the edk2 packages in the Debian and Ubuntu
distributions. A few times over the years I've had users report
regressions that turned out to be due to upstream build flag
changes. I wonder if it would be possible for upstream to communicate
such changes - either via release notes or, where possible, as
build-time checks - to us downstream projects. Here are the examples I
recall:

> commit 4de8d61bcec02a13ceed84f92b0cf3ea58adf9c5
> Author: Gerd Hoffmann <kraxel@redhat.com>
> Date:   Wed Dec 15 12:39:20 2021 +0100
> 
>     OvmfPkg: rework TPM configuration
> 

For this I needed to change -DTPM_ENABLE=TRUE to -DTPM2_ENABLE=TRUE. I
feel like a build-time check would've been a good way to communicate
this one, e.g.:

#ifdef TPM_ENABLE
#error TPM_ENABLE has been renamed to TPM2_ENABLE
#endif

> commit 1631bb26ae991e530d3c96fe3161ea15144b358e
> Author: Gary Lin <glin@suse.com>
> Date:   Mon Jun 10 14:55:09 2019 +0800
> 
>     OvmfPkg/README: Update the network build flags
>     
>     The following network build flags changed due to the inclusion of
>     NetworkPkg/Network.fdf.inc.
>
Similar to the above, I needed to deal with this by setting
-DNETWORK_HTTP_BOOT_ENABLE=TRUE instead of -DHTTP_BOOT_ENABLE=TRUE. It
seems like maybe an #error on the removed #define could've helped
here as well.

And finally:

> commit 57783adfb579da32b1eeda77b2bec028a5e0b7b3
> Author: Michael D Kinney <michael.d.kinney@intel.com>
> Date:   Tue Jul 26 12:40:00 2022 -0700
>
>     OvmfPkg: Change default to disable MptScsi and PvScsi

With this types of change, it would be useful just to know this is
coming and why so I can decide to either notify users of the change
and/or override the default to avoid the regression. Would it be
possible to describe such changes in the release notes at
https://github.com/tianocore/edk2/releases - and, if not urgent,
perhaps #warn of deprecation in the source for one stable release
cycle?

  -dann