hdl: Store build artifacts in a timestamped directory, with reports

FPGA build artifacts are now copied to a <rev>x<size>-<timestamp>
directory. The .rbf, SHA256 & MD5 checksums, and reports/summaries are
copied into this directory.

This is intended to help keep files with their associated FPGA images, as
items in work/ are clobbered with each invocation of build_bladerf.sh.