kbuild: fix section mismatch check for vmlinux

vmlinux does not contain relocation entries which is
used by the section mismatch checks.
Reported by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>

Use the individual objects as inputs to overcome
this limitation.
In modpost check the .o files and skip non-ELF files.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>