Support ZBOOT kernel images
As we move towards UEFI on more devices, we want to use systemd-boot and kernel images built with CONFIG_ZBOOT. However, these images aren't compatible with existing Android bootloaders. As a result, we must install both the old vmlinuz image for old bootloaders, and the fancy new self-extracting EFI image.
But, they don't both fit in the /boot partition (which already contains multiple copies of the kernel when appending the DTB for example).
To resolve this, support reading the vmlinuz image from /lib/modules/$KERNELRELEASE/vmlinuz, this is standard practise for other distributions (Arch Linux) already, and allows kernel packages to free up space in /boot for the ZBOOT kernel image.
The -dtb appended kernel is still installed to /boot, as is the boot.img for Android devices. This could break extlinux booting, hence it's up to the kernel package to choose where the vmlinuz image is installed.