Make it clear that KERNEL_ALT_IMAGETYPE is intended to allow building an
additional image type.
(From yocto-docs rev: d2f51d310028dfa50584f7dc04ea3627d14d3f1e)
Signed-off-by: Paul Eggleton <paul.eggleton@microsoft.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>