Don't rely on dlopen for EBL backends

Don't rely on dlopen for EBL backends

- EBL was modified to avoid dlopen and instead relies on all supported
  backends being linked in.

- backends/libebl_static_pic.a and libcpu/libcpu_static_pic.a contain
  selected backends to be linked statically together with libebl.a

- libdw/libdw_static_pic.a is a minimal subset of libdw containing symbols
  that the above two archives reference.  Binaries that don't link libdw
  but use libebl need to link in libdw_static_pic.  This only for internal
  uses, externally we only support libdw.

- Test cases that excercise EBL on unsupported platforms were disabled

Signed-off-by: Mark Wielaard <mjw@redhat.com>