PPC: Fail configure when libfdt is not available

We have several targets in the PPC tree now that basically require libfdt
to function properly, namely the pseries and the e500 targets. This dependency
will rather increase than decrease in the future, so I want to make sure
that people building shiny new 1.0 actually have libfdt installed to get
rid of a few ifdefs in the code.

Warning: This patch will likely make configure fail for people who don't
select their own --target-list, but don't have libfdt development packages
installed. However, we really need this new dependency to move on.

Signed-off-by: Alexander Graf <agraf@suse.de>

---

v1 -> v2:

  - no paranthesis
  - no fdt check for config_pseries
  - add . in error message
