libftdi: C++ bindings need boost

According to libftdi configure.in:

"""
dnl libftdi C++ wrapper. Needs boost.
[...]
        if test "x$HAVE_BOOST" != "xyes"; then
            AC_MSG_ERROR(Sorry, we need the boost library for the C++ wrapper)
        fi
"""

And indeed, if you enable BR2_PACKAGE_LIBTFDI_CPP but don't have Boost
enabled, the libfdipp library is not built. To fix this, this commit
changes BR2_PACKAGE_LIBTFDI_CPP to select Boost.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>