ref-manual: variables: provide no-match example for COMPATIBLE_MACHINE

COMPATIBLE_MACHINE is used to forbid the use of a recipe or its packages
for a specific set of machines.

In some cases, it may make more sense to have the logic inverted and
have the recipe always forbidden except for hand-picked machines. Such
could be the case for pieces of software that only support some
architectures. In that scenario, it is sometimes a bit easier on the eye
and for maintenance to use the OVERRIDES mechanism but for that, a
default should be set.

COMPATIBLE_MACHINE:aarch64 = "^(aarch64)$"
COMPATIBLE_MACHINE:mips64 = "^(mips64)$"

wouldn't do much because if COMPATIBLE_MACHINE isn't set, the recipe is
assumed compatible and therefore, if no default is provided we enter
that case.

Hence, we need to add

COMPATIBLE_MACHINE = "^$"

as default so that it only matches the empty string, which isn't
possible for MACHINEOVERRIDES.

Cc: Quentin Schulz <foss+yocto@0leil.net>
(From yocto-docs rev: d66b53eebe5b2320ce12926b309e48c9e32523f3)

Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Reviewed-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>

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