customize: fix copy paths

Closes #849, #1135

customize.mk used to copy files with a wrong command, resulting in one
directory level in excess. For example, source/etc/myfile would have
produced /etc/etc/myfile in the target filesystem instead of /etc/myfile.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>