libftdi: add explicit dependencies for host variant
This patch also removes an incorrect dependency between host-libftdi and
host-boost when BR2_PACKAGE_LIBTFDI_CPP is set.
Signed-off-by: Julien Floret <julien.floret@6wind.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>