vivado: resolve install path from ~/skills_env.md instead of hardcoding D:\Xilinx

The Vivado install root differs per machine (C:\Xilinx here, D:\Xilinx on the
box these skills were written on), so every hardcoded path was already wrong
on this machine.

- new fpga_flash/scripts/vivado_env.py: resolves --vivado-dir > env var >
  VIVADO_ROOT (falling back to VIVADO_PATH) from ~/skills_env.md
- gen_mcs / program_flash / check_flash_csv: --vivado-dir now optional
- fpga_update/gen_update_mcs.py: same precedence for --vivado
- fpga_flash / smod_atsc_dev / vivado-prj-create SKILL.md: document the
  lookup instead of naming a drive letter
- skills_env template: VIVADO_ROOT/VIVADO_PATH are now blanks to fill in,
  so a new machine can't inherit a wrong default

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

fpga_update/scripts/gen_update_mcs.py

@@ -34,7 +34,41 @@ IMPL = os.path.join(REPO, "my_exciter.runs", "impl_1")
 POST_BITSTREAM = os.path.join(TCL_DIR, "post_bitstream.tcl")
 GEN_RU = os.path.join(TCL_DIR, "runScript_GenRu.tcl")
 PERL_SCRIPT = os.path.join(TCL_DIR, "MakeSpiFlashProgrammerMcsFiles_backup.pl")
-DEFAULT_VIVADO = r"D:\Xilinx\Vivado\2018.1\bin\vivado.bat"
+
+
+def _env_lookup(key, default=None):
+    """Machine-local values (paths, IPs) live in ~/skills_env.md, never in git.
+    An OS environment variable of the same name overrides the file."""
+    v = os.environ.get(key)
+    if v:
+        return v
+    path = os.path.join(os.path.expanduser("~"), "skills_env.md")
+    try:
+        with open(path, encoding="utf-8") as f:
+            for line in f:
+                m = re.match(rf"\s*{re.escape(key)}\s*=\s*(.+?)\s*(?:#.*)?$", line)
+                if m:
+                    val = m.group(1).strip()
+                    if val and not val.startswith("<"):
+                        return val
+    except OSError:
+        pass
+    return default
+
+
+def resolve_vivado():
+    """Vivado's install path differs per machine -- never hardcode it."""
+    exe = _env_lookup("VIVADO_PATH")
+    if exe:
+        return exe
+    root = _env_lookup("VIVADO_ROOT")
+    if root:
+        return os.path.join(root, "bin", "vivado.bat")
+    sys.exit(
+        "Vivado path not configured. Add `VIVADO_PATH = <...>/bin/vivado.bat` to "
+        "~/skills_env.md, set it as an env var, or pass --vivado."
+    )
+
 
 # runScript_GenRu.tcl hardcodes this name.
 BASE_MCS = "atsc_exciter.mcs"
@@ -66,10 +100,12 @@ def first_ela(path):
 
 def main():
     ap = argparse.ArgumentParser(description=__doc__, formatter_class=argparse.RawDescriptionHelpFormatter)
-    ap.add_argument("--vivado", default=DEFAULT_VIVADO)
+    ap.add_argument("--vivado", default=None,
+                    help="path to vivado.bat; defaults to VIVADO_PATH from ~/skills_env.md")
     ap.add_argument("--tag", help="rename the result to <tag>_update.mcs so it survives the next run")
     ap.add_argument("--keep-byproducts", action="store_true")
     args = ap.parse_args()
+    args.vivado = args.vivado or resolve_vivado()
 
     for path, what in ((args.vivado, "vivado"), (POST_BITSTREAM, "post_bitstream.tcl"),
                        (GEN_RU, "runScript_GenRu.tcl"), (PERL_SCRIPT, "perl script")):