scripts: Add simple GPIO loopback script

This uses gpioset/gpioget to set an output and read back the value from
an input.

scripts/simple_gpio_loopback.sh

+#!/bin/sh
+
+GPIO_OUT=$(gpiofind "$1")
+GPIO_IN=$(gpiofind "$2")
+RETVAL=0
+
+print_help() {
+    echo "Usage: simple_gpio_loopback.sh [Output] [Input]"
+    echo ""
+    echo "Example:"
+    echo "    simple_gpio_loopback.sh GPIO13 GPIO12"
+}
+
+if [ -z "$1" ]; then
+    echo "ERROR: Output not set ($1 / $2)"
+    print_help
+    exit 1
+elif [ -z "$2" ]; then
+    echo "ERROR: Input not set ($1 / $2)"
+    print_help
+    exit 1
+elif [ -z "$GPIO_OUT" ]; then
+    echo "ERROR: Lookup of Output failed ($1 / $2)"
+    print_help
+    exit 1
+elif [ -z "$GPIO_IN" ]; then
+    echo "ERROR: Lookup of Input failed ($1 / $2)"
+    print_help
+    exit 1
+fi
+
+echo "OUT: $GPIO_OUT - IN: $GPIO_IN"
+
+# shellcheck disable=2086
+gpioset --mode=signal $GPIO_OUT=1 &
+PID="$!"
+
+sleep 0.5
+
+# shellcheck disable=2046 disable=2086
+if ! [ 1 -eq $(gpioget $GPIO_IN) ]; then
+    echo "ERROR: Input value does not match ($1 / $2)"
+    RETVAL=1
+fi
+
+kill $PID
+
+# shellcheck disable=2086
+gpioset $GPIO_OUT=0
+
+exit $RETVAL