libfdt: Add necessary header padding in fdt_create()

At present fdt_create() will succeed if there is exactly enough space to
put in the fdt header.  However, it sets the off_mem_rsvmap field, a few
bytes past that in order to align the memory reservation block.

Having block pointers pointing past the end of the fdt is pretty ugly, even
if it is just a transient state.  Worse, if fdt_resize() is called at
exactly the wrong time, it can end up accessing data past the blob's
allocated space because of this.

So, correct fdt_create() to ensure that there is sufficient space for the
alignment padding as well as the plain header.  For paranoia, also add a
check in fdt_resize() to make sure we don't copy data from outside the
blob's bounds.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>