Add buttons_arty target

[duck2]

I tried to add a buttons_arty test by adding an arty target to xc7/tests/buttons and creating the required arty.pcf and buttons_arty.v files, but building it failed with this error:

PCF constraint "set_io in[0] A8" from line 2 constraints pad A8 which is not in available pad map:
A9
C2
D10
E3
T10

This seems to because of the way the arty device is defined:

add_xc7_device_define_type(
...
  ROI_DIR ${PRJXRAY_DB_DIR}/artix7/harness/arty-a7/uart
...
)

In the prjxray repository, there is also swbut and pmod harnesses, but it doesn't seem to be possible to specify the harness in the test. How to go about this? Make a harness for arty including IO for both uart and buttons or try and generalize test targets?

[acomodi]

@duck2 I believe the easiest way to add a buttons target for arty and use the swbut harness is to create another directory in https://github.com/SymbiFlow/symbiflow-arch-defs/tree/master/xc7/archs/artix7/devices and call it something like: "xc7a50t-arty-swbut-roi-virt".

Than in there use the same CMakeLists.txt as for the other arty device and change the ROI to be used.

[acomodi]

Then, in the arty_button tests you will have to specify the correct I/Os in the arty.pcf files as they appear in the swbut ROI and add the arty_buttons target to the CMakeLists.txt using the newly added device

[duck2]

#1006