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core/pkg-generic: host variants inherits target's override-srcdir

When a package has both a target and a host variants, and there is an
override-srcdir set for the target variant, the host variant should
inherit the target's override-srcdir, unless explicitly set, like we do
for all other target-variant properties.

However, do not override it if expressly set to empty (i.e. when the
user wants to override only the target variant).

Also, as we do for the other variables, the target variant never
inherits from the host variant.

Reported-by: Mike <mikez@OpenPlayer.org>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Reviewed-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Tested-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>