scripts: objdiff: improve path flexibility for record command

Prior to this commit, scripts/objdiff expected to be run at the top
directory and only the relative path of objects.

This commit provides more flexibility in terms of object path:

[1] scripts/objdiff can be run in any directory

For example,

  $ scripts/objdiff record init/main.o

and

  $ cd init; ../scripts/objdiff record main.o

produce the same result.

[2] Support absolute path for objects

  $ scripts/objdiff record /home/foo/bar/linux/init/main.o

work as well.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Michal Marek <mmarek@suse.cz>