classes/kernel*: allow disabling symlink creation

classes/kernel*: allow disabling symlink creation

Allow setting any of the *_LINK_NAME variables to empty string in order
to disable creating symlinks for kernel artifacts, as you can already
for filesystem images with IMAGE_LINK_NAME. Additionally, for the image
type named symlinks, add a KERNEL_IMAGETYPE_SYMLINK boolean variable
which you can set to 0 to disable those symlinks as well.

Signed-off-by: Paul Eggleton <paul.eggleton@microsoft.com>