kernel bbclass: recreate uImage unless KEEPUIMAGE is set

The intent of the uImage code in this class includes the following

1) be able to specify custom load addresses without needing to patch the kernel
2) add better information to the uImage description field

The current state is a NOP anyway, the kernel will always build a uImage when you tell it to 'make uImage'.

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>