scripts: objdiff: support directories for the augument of record command

For example,

  $ scripts/objdiff record init drivers/usb

disassembles all the objects under init and drivers/usb directories.

This feature would be useful when we change various files under the
specific directory.

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>