linux: select BR2_PACKAGE_HOST_UBOOT_TOOLS instead of BR2_LINUX_KERNEL_UBOOT_IMAGE

linux: select BR2_PACKAGE_HOST_UBOOT_TOOLS instead of BR2_LINUX_KERNEL_UBOOT_IMAGE

A long time ago, the blind config option BR2_LINUX_KERNEL_UBOOT_IMAGE
was introduced to be able to trigger the linux -> host-uboot-tools
dependency. Back in those days, there was no user-configurable
BR2_PACKAGE_HOST_UBOOT_TOOLS.

Now, however, it is possible to select a custom kernel image name that
needs uboot-tools, and manually enable BR2_PACKAGE_HOST_UBOOT_TOOLS. In
this case, however, the linux -> host-uboot-tools is missed and the
build is not reproducible. An example of such a situation is the
upcoming CI40 defconfig.

As a solution, remove BR2_LINUX_KERNEL_UBOOT_IMAGE entirely. Instead,
just select BR2_PACKAGE_HOST_UBOOT_TOOLS and add the dependency if it
is selected.

Note that this may introduce a redundant dependency in case the user
selected BR2_PACKAGE_HOST_UBOOT_TOOLS for some other reason (e.g. to
be able to generate a U-Boot environment to include in the image, while
the kernel is built as a zImage). However, the redundant dependency
shouldn't hurt much.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Abhimanyu Vishwakarma <abhimanyu.v@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>