configure: Check for libfdt version 1.4.0

Some recent patches require a function from libfdt version 1.4.0,
so we should check for this version during the configure step
already. Unfortunately, there does not seem to be a proper #define
for the version number in the libfdt headers. So alternatively,
we check for the availability of the required function
fdt_get_property_by_offset() instead instead.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
