avoid libfdt checks adding full paths to pkg-config

avoid libfdt checks adding full paths to pkg-config

The checks for libfdt try dependency() first which would only work if
a pkg-config would be present but libfdt has none.
Then it probes for the lib path itself via cc.find_library.

But later it adds the result of either probe to ext_deps which ends up
in build and also the resulting pkg-config to contain toolchain versioned
paths in Libs.private like:
  /usr/lib/gcc/x86_64-linux-gnu/9/../../../x86_64-linux-gnu/libfdt.so
which obviously breaks on toolchain updates.

In general libs used multiple times - ipn3ke + ifpga in this case - are
checked centrally in config/meson.build so move it there and fix the
adding of dependencies to not use the full file path.

The result is libfdt in pkg-config now showing up as:
  Libs.private: -pthread -lm -ldl -lnuma -lfdt -lpcap

Fixes: e1defba4cf66 ("raw/ifpga/base: support device tree")
Cc: stable@dpdk.org
Signed-off-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>
Reviewed-by: Luca Boccassi <bluca@debian.org>
Reviewed-by: Bruce Richardson <bruce.richardson@intel.com>