archiver.bbclass: Fix work-shared checking for kernel recipes

Source dir can be a symbolic link in some BSP's linux kernel recipe
which points to work-shared path (like linux-fslc in meta-freescale).
Change to use os.path.realpath() in order to get real path of
source dir.

Signed-off-by: Benjamin Szőke <egyszeregy@freemail.hu>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>