Makefile: refactor ubootrelease target

Instead of duplicating the contents of the filechk_uboot.release
variable, use it directly.

This is preparation for the next patch which will modify
filechk_uboot.release, and reflects what the linux kernel does
nowadays:

kernelrelease:
	@$(filechk_kernel.release)

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>