applet.interface.jtag_openocd: new applet. Our first nMigen applet!

applet.debug.mips: fix typo. NFC.

applet.debug.mips: make sure DMAAcc bit is set before writing ADDRESS. On BMIPS ADDRESS is writable if NoDMA=0, on Lexra ADDRESS is only writable if DMAAcc=1.

applet.debug.mips: restrict Rocc handshaking to EJTAG >2.0. BMIPS seems to support it, but Lexra does not, even though both are EJTAG 2.0, so disable it on EJTAG 2.0 completely. Also, print every CONTROL bit, instead of only the overridden one.

support.bitstruct: use __slots__. Mostly to ensure setting nonexistent attributes fails, but this also makes it a bit more efficient.

applet.debug.mips: log scanned address length.

software,firmware: fix factory programming. The new device detection code (introduced in 3ff1f20a) does not work if the device reenumerates with Glasgow VID/PID, so make it enumerate with the default Cypress VID/PID.

applet.audio.yamaha_opx: OPL3: set CHA=CHB=1 on reset.

applet.audio.yamaha_opx: add `run` subcommand.

applet.audio.yamaha_opx: fix handling of oversized files.