Add SVD export capability to Builder (csr_svd parameter) and targets (--csr-svd argum...

Add SVD export capability to Builder (csr_svd parameter) and targets (--csr-svd argument) and fix svd regression.

This allows generating SVD export files during the build as we are already doing for .csv or .json.

Use with Builder:
builder = Builder(soc, csr_svd="csr.svd")

Use with target:
./arty.py --csr-svd=csr.svd