Makefile: clarify a comment

Makefile: clarify a comment

os user <gnusercn@gmail.com> writes:
From the last comment, arch makefile will override vmlinux. It seems
vmlinux will not be checked by `make'. But from my test, although
`all:' will be re-defined in arch Makefile (ARM arch), vmlinux will
still be checked and the commands associated will be executed. Should
we use another word instead of "overridden"?

Reported-by: os user <gnusercn@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>