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Mirrors of meta-arm may focus their development on a small subset of
MACHINEs so provide the option to restrict the boards that are built on
CI using the variable BUILD_ENABLE_REGEX. If set, it conditionally
enables builds; if unset there is no change in behavior.

This variable could be overridden in a scheduled build, to e.g. build
all the MACHINEs weekly.
Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>