Correct way to set DRIDRIVERS and GALLIUMDRIVERS from mesa bbappend

[Mike Crowe <mac@mcrowe.com>]

I'm building for a specific chip and therefore don't wish to waste time and
electricity building and disk space on the target installing unwanted mesa
drivers. However, mesa.inc contains:

 GALLIUMDRIVERS = "swrast"
 GALLIUMDRIVERS:x86-x32 = ""
 GALLIUMDRIVERS:append:x86:class-target = ",i915,iris,crocus"
 GALLIUMDRIVERS:append:x86-64:class-target = ",i915,iris,crocus"

and mesa_21.3.0.bb contains:

 DRIDRIVERS ??= ""
 DRIDRIVERS:append:x86:class-target = ",r100,r200,nouveau,i965"
 DRIDRIVERS:append:x86-64:class-target = ",r100,r200,nouveau,i965"

I'm unable to find a way to override these values. Using (for example):

 DRIDRIVERS:forcevariable = ""
 GALLIUMDRIVERS:forcevariable = "swrast"

doesn't work because the append still happens after the forcevariable
override takes effect. :(

Is there a way that I can override GALLIUMDRIVERS and DRIDRIVERS with my
own values for x86 and x86-64 without modifying oe-core itself?

If not, should the oe-core recipe being using something like:

 GALLIUMDRIVERS_DEFAULT = "swrast"
 GALLIUMDRIVERS_DEFAULT:x86-x32 = ""
 GALLIUMDRIVERS_DEFAULT:append:x86:class-target = ",i915,iris,crocus"
 GALLIUMDRIVERS_DEFAULT:append:x86-64:class-target = ",i915,iris,crocus"
 GALLIUMDRIVERS ?= "${GALLIUMDRIVERS_DEFAULT}"

and similar for DRIDRIVERS to support this?

Thanks.

Mike.