kernel: Pull uImage generation into separate class
Pull the uImage image format generation from kernel.bbclass into
a separate kernel-uimage.bbclass. Introduce new KERNEL_CLASSES
variable, which allows registration of additional classes which
implement new kernel image types. The default value of is to
register kernel-uimage to preserve the original behavior.
Signed-off-by: Marek Vasut <marex@denx.de> Cc: Richard Purdie <richard.purdie@linuxfoundation.org> Cc: Koen Kooi <koen@dominion.thruhere.net> Cc: Paul Eggleton <paul.eggleton@linux.intel.com> Cc: Ross Burton <ross.burton@intel.com> Cc: Bruce Ashfield <bruce.ashfield@windriver.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
