sunxi-boards: new package

[Peter: depend on arm, wrap help, change to _FEX_FILE option, error on empty]
Signed-off-by: Carlo Caione <carlo.caione@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>