default to 'boot' as boot partition label
default to 'boot' as boot partition label

Defaulting to the $MACHINE may lead to failure if the name was
longer than the limit of allowed volume label of fat (11 chars).

Signed-off-by: Marius Kriegerowski <8400727a4fb4aa6abe029d4260bb5b32e80c15d3@gmail.com>

linux-sunxi/meta-sunxi,linux-sunxi/meta-sunxi,linux-sunxi/meta-sunxi