linux: fix appended dtb handling of pre/post v3.8-rc1

The $(wildcard ) doesn't work for LINUX_APPEND_DTB, because the .dtb
doesn't exist yet at that point.

Also factor the common part out.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>