Replace init_mss with tiny Python script (#655)

userspace/root/usr/comma/init_mss.py

@@ -0,0 +1,10 @@
+#!/usr/bin/python3
+import signal
+
+# The on-chip Wi-Fi firmware runs in the SDM845 modem subsystem.
+# Opening this device boots it and holds a reference; the kernel shuts it
+# down when the last reference is released. Keep it open for this service's
+# lifetime so Wi-Fi stays available.
+# https://github.com/commaai/agnos-kernel-sdm845/blob/8b0e4d289b246a5cc1b0484b25a3f9f8d4772544/drivers/soc/qcom/subsystem_restart.c#L1215-L1251
+with open("/dev/subsys_modem", "rb"):
+  signal.pause()

userspace/root/usr/lib/systemd/system/init_sys_mss.service

@@ -2,7 +2,7 @@
 Description=Boot the modem subsystem, which runs the Wi-Fi firmware
 
 [Service]
-ExecStart=/usr/sbin/init_mss
+ExecStart=/usr/bin/python3 /usr/comma/init_mss.py
 Restart=on-failure
 
 [Install]