flex: Replace uninative loader path

The Makefile used for flex-ptest can pick up the path to the uninative
loader through BUILD_LDFLAGS. This includes the full path to the
uninative loader, which is not reproducible. Replace it with /bin/false.
It doesn't appear as if these native programs are used in the test
suites and if there are likely to be other problems related to building
them using the BUILD_* flags.

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>