pkg-virtual: fix host dependencies handling

pkg-virtual: fix host dependencies handling

If $(BR2_PACKAGE_HAS_HOST_FOO) is defined, then the pkg-virtual
infrastructure will check if $(BR2_PACKAGE_PROVIDES_HOST_FOO) is not
empty.

But later, $(BR2_PACKAGE_HOST_FOO_DEPENDENCIES) will be set from
$(BR2_PACKAGE_PROVIDES_FOO), ignoring $(BR2_PACKAGE_PROVIDES_HOST_FOO).

So fix this discrepancy by setting $(BR2_PACKAGE_HOST_FOO_DEPENDENCIES)
from $(BR2_PACKAGE_PROVIDES_FOO) only if $(BR2_PACKAGE_PROVIDES_HOST_FOO)
is empty.

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
[Thomas: add comments suggested by Arnout.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>