libbsd: enable package on ARM

Rework the architecture dependency by adding a
BR2_PACKAGE_LIBBSD_ARCH_SUPPORTS blind option.

[Thomas: slightly reword commit log.]

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>