zynq: use /mnt for bitfile storage

This fixes a problem with current zedboard buildroot images whereby the
bitfile is too large for ramfs storage.
zynq: make "masked" tb_*_files the default

The tb_verilog_files/tb_vhdl_files mechanism was added to resolve issues
where huge sets of synth-specific HDL were getting added to simulations
and causing errors. This commit makes using these special-purpose file
lists the default for zynq projects.