cmake: Rename kernel_elf to zephyr_final

 * KERNEL_ELF was easily confused with KERNEL_ELF_NAME and KERNEL_NAME.
 * kernel_elf as the name of the binary indicates it somehow only
   contains the kernel, which is not correct.

Rename to zephyr_final as this is the reality: Zephyr elf has been
linked again (a number of times) due to generated kernel files.

Signed-off-by: Mark Ruvald Pedersen <mped@oticon.com>