add host arch detection and Kconfig BR2_HOSTARCH

This will allow to install binary package only if they are supported by the
host. As example Atmel SAM-BA (x86 only).

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>