New LF edge detection algorithm + lowpass filter

This is a new LF edge detection algorithm for the FPGA.

- It uses a low-pass IIR filter to clean the signal
(see https://fail0verflow.com/blog/2014/proxmark3-fpga-iir-filter.html)
- The algorithm is able to detect consecutive peaks in the same
  direction
- It uses an envelope follower to dynamically adjust the peak thresholds
- The main threshold used in the envelope follower can be set from the ARM side

fpga/lf_edge_detect.v,
fpga/lp20khz_1MSa_iir_filter.v,
fpga/min_max_tracker.v: New file.

fpga/lo_edge_detect.v, fpga/fpga_lf.v: Modify accordingly.

armsrc/apps.h (FPGA_CMD_SET_USER_BYTE1,
FPGA_CMD_SET_EDGE_DETECT_THRESHOLD): New FPGA command.
fpga/fpga_lf.v: Modify accordingly/Add a 8bit user register.

fpga/fpga_lf.bit: Update accordingly.

fpga/tests: New directory for testbenches

fpga/tests/Makefile: New file. It compiles the testbenches
and runs all the tests by default (comparing with the golden output)

fpga/tests/tb_lp20khz_1MSa_iir_filter.v,
fpga/tests/tb_min_max_tracker.v,
fpga/tests/tb_lf_edge_detect.v: New testbenches

fpga/tests/plot_edgedetect.py: New script to plot the results from
the edge detection tests.

fpga/tests/tb_data: New directory for data and golden outputs