quad: add unix virtual quad stubs

quad/.gitignore

quad/Makefile

quad/executable.mk

quad/src/virt_quad/Makefile

quad/src/virt_quad/hw_impl_unix.c

quad/src/virt_quad/hw_impl_unix.h

quad/src/virt_quad/hw_impl_unix_axi_timer.c

quad/src/virt_quad/hw_impl_unix_global_timer.c

quad/src/virt_quad/hw_impl_unix_i2c.c

quad/src/virt_quad/hw_impl_unix_mio7_led.c

quad/src/virt_quad/hw_impl_unix_pwm_input.c

quad/src/virt_quad/hw_impl_unix_pwm_output.c

quad/src/virt_quad/hw_impl_unix_system.c

quad/src/virt_quad/hw_impl_unix_uart.c

quad/src/virt_quad/main.c