Update rpi3 u-boot to detect newer hardware
Update rpi3 u-boot to detect newer hardware

Update the u-boot image included in the mkimage-rpi3 image to support
detecting newer hardware versions and setting the fdtfile variable
accordingly
Shallow clone the u-boot repository during docker build to improve build
efficiency

Signed-off-by: Richard Connon <320bca71fc381a4a025636043ca86e734e31cf8b@connon.me.uk>

yankcrime/linuxkit,justincormack/linuxkit,rn/linuxkit,djs55/linuxkit,konstruktoid/linuxkit,djs55/linuxkit,konstruktoid/linuxkit,linuxkit/linuxkit,yankcrime/linuxkit,JohnnyLeone/linuxkit,JohnnyLeone/linuxkit,furious-luke/linuxkit,justincormack/linuxkit,deitch/linuxkit,rn/linuxkit,rn/linuxkit,furious-luke/linuxkit,konstruktoid/linuxkit,justincormack/linuxkit,konstruktoid/linuxkit,furious-luke/linuxkit,yankcrime/linuxkit,JohnnyLeone/linuxkit,rn/linuxkit,furious-luke/linuxkit,djs55/linuxkit,furious-luke/linuxkit,deitch/linuxkit,justincormack/linuxkit,yankcrime/linuxkit,justincormack/linuxkit,JohnnyLeone/linuxkit,JohnnyLeone/linuxkit,rn/linuxkit,deitch/linuxkit,djs55/linuxkit,djs55/linuxkit,konstruktoid/linuxkit,yankcrime/linuxkit,deitch/linuxkit,justincormack/linuxkit,konstruktoid/linuxkit,yankcrime/linuxkit,linuxkit/linuxkit,deitch/linuxkit,linuxkit/linuxkit,linuxkit/linuxkit,linuxkit/linuxkit