Add flash.sh script

This script simplifies the process of erasing the target disk and
flashing `sdcard.img` onto it.

flash.sh

@@ -0,0 +1,20 @@
+#!/bin/sh
+# vim: set sw=4 ts=4 sts=4 et :
+
+dev=$(readlink -f "$1")
+wipefs -fa "${dev}"
+pv images/sdcard.img \
+    | dd of="${dev}" bs=1M conv=sparse oflag=sync
+sync; sync; sync
+sleep 1
+for p in /sys/class/block/${dev##*/}/*/partition; do
+    [ -f "${p}" ] || continue
+    case $(cat "${p}") in
+    1|2)
+        ;;
+    *)
+        p=${p%/*}
+        wipefs -fa /dev/${p##*/}
+        ;;
+    esac
+done