kernel bbclass: Recreate uImage only when KEEPUIMAGE != "yes"
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'.
weakly Set KEEPUIMAGE to 'yes' in default-distrovars.inc which preserve the
current OE-Core behavior. Machines which are being ported from oe.dev and need to
regenerate uImage can set this to be empty
