sunxi: use fwtool for checking sdcard images

sunxi: use fwtool for checking sdcard images

To achieve this, device tree compatible string was used as boardname and
the value of it will be checked against supported_devices list.

It should be noted that we do not distinguish between
sun5i-a13-olimex-som and sun5i-a13-olinuxino as they share the same dts
file.

The other thing is that we need to gunzip the generated firmware to do
fwtool check.

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>