arm/frdm_k64f: only leave space for security in XIP images

This board needs 16 bytes to be written with a specific value when the
target boots. This is only necessary when running a XIP image which
exists around those 16 bytes.

Change-Id: Ifd26b3842f09137765d9c7d1678476bfda8a563f
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>
Signed-off-by: Vlad Lungu <vlad.lungu@windriver.com>