Document how to run config-pin at boot

Author: pdp7

doc/run_config-pin_during_startup.md

@@ -0,0 +1,55 @@
+## Create file `/usr/bin/enable-i2c-pins.sh`
+ **sudo vi /usr/bin/enable-i2c-pins.sh**
+```
+#!/bin/bash
+
+config-pin p9.17 i2c
+config-pin p9.18 i2c
+```
+**sudo chmod 755 /usr/bin/enable-i2c-pins.sh**
+
+## Create file `/lib/systemd/system/enable-i2c-pins.service`
+**sudo vi /lib/systemd/system/enable-i2c-pins.service**
+```
+[Unit]
+Description=Enable I2C pins
+After=generic-board-startup.service
+
+[Service]
+Type=simple
+ExecStart=/usr/bin/enable-i2c-pins.sh
+
+[Install]
+WantedBy=multi-user.target
+```
+## Enable the new systemd service
+**sudo systemctl daemon-reload**
+**sudo systemctl enable enable-i2c-pins.service**
+```
+Created symlink /etc/systemd/system/multi-user.target.wants/enable-i2c-pins.service → /lib/systemd/system/enable-i2c-pins.service.
+```
+
+## Reboot and test
+**sudo systemctl status enable-i2c-pins.service**
+```
+debian@beaglebone:~$ sudo systemctl status enable-i2c-pins.service
+● enable-i2c-pins.service - Enable I2C pins
+   Loaded: loaded (/lib/systemd/system/enable-i2c-pins.service; enabled)
+   Active: inactive (dead) since Wed 2017-04-19 06:35:06 UTC; 48s ago
+  Process: 649 ExecStart=/usr/bin/enable-i2c-pins.sh (code=exited, status=0/SUCCESS)
+ Main PID: 649 (code=exited, status=0/SUCCESS)
+
+Apr 19 06:35:05 beaglebone sudo[664]: root : TTY=unknown ; PWD=/ ; USER=root ; COMMAND=/bin/bash -c echo i2c > /sys/devices/platform/ocp/ocp:P9_17_pinmux/state
+Apr 19 06:35:05 beaglebone sudo[664]: pam_unix(sudo:session): session opened for user root by (uid=0)
+Apr 19 06:35:05 beaglebone sudo[664]: pam_unix(sudo:session): session closed for user root
+Apr 19 06:35:06 beaglebone sudo[677]: root : TTY=unknown ; PWD=/ ; USER=root ; COMMAND=/bin/bash -c echo i2c > /sys/devices/platform/ocp/ocp:P9_18_pinmux/state
+Apr 19 06:35:06 beaglebone sudo[677]: pam_unix(sudo:session): session opened for user root by (uid=0)
+Apr 19 06:35:06 beaglebone sudo[677]: pam_unix(sudo:session): session closed for user root
+```
+
+**config-pin -q p9.17 && config-pin -q p9.18**
+```
+debian@beaglebone:~$ config-pin -q p9.17 && config-pin -q p9.18
+P9_17 Mode: i2c
+P9_18 Mode: i2c
+```