util/abuild: Identify abuild builds with an env variable

This environment variable can be used to identify when the build is
running from abuild. This can control things like whether or not the
payloads will pull down a new version from git.
This is important on the builders because the network can't be accessed,
but also it'd be unexpected to change the state of the tree when running
abuild locally.

Change-Id: I03a29aeff655ba7067b505b4e26d5b0f4157c67f
Signed-off-by: Martin Roth <gaumless@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/87366
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Matt DeVillier <matt.devillier@gmail.com>

util/abuild/abuild

@@ -29,6 +29,9 @@ TESTRUN="${TESTRUN_DEFAULT}"
 
 export KCONFIG_OVERWRITECONFIG=1
 
+# Identify this as a coreboot autobuild
+export COREBOOT_ABUILD=1
+
 # path to payload. Should be more generic
 PAYLOAD=/dev/null