Build the uImage file using the kernel build system only when
it is really required, which is only in case KEEPUIMAGE == yes.
Otherwise, just build zImage, since the Yocto build system will
handle the uImage generation for us.

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>

Separate the function which prepares the kernel for packing into
uImage into separate class, so this function can be reused by the
fitImage class.

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>

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>