systems: add minimal de0 nano board support

Supported peripherals are:
SDRAM, uart and debug_if.
Highest on TODO list is to add SPI, I2C and simulation support.
Lower on the list is to bring in support for LCD and touchscreen
add-on board.
Merge branch 'de0_nano' of https://github.com/skristiansson/orpsoc-cores
add .gitignore
cores: add altera_virtual_jtag core

Used as a wrapper to altera virtual jtag by advanced debug sys,
but isn't limited to that.
de0_nano: use common altera_virtual_jtag core