Install kernels to /boot

Install kernels to /boot

If users want to use XBOOTLDR partitions, then we have to put the
kernels in a separate location from the ESP stuff. Currently we put
everything in /efi when building the image, which means that users
don't have a way to specify that the kernels should be put in an
XBOOTLDR partition.

Let's fix this by installing kernels to /boot so that users can
populate an XBOOTLDR partition by simply using CopyFiles=/boot:/
in a repart xbootldr partition definition.