[POWERPC] bootwrapper: Add ft_find_device_rel().

Add a function to look up a relative, rather than absolute, path name.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Acked-by: Mark A. Greer <mgreer@mvista.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>