adb: Add "wait-for-sideload" command

 * Waits for connected device to enter sideload mode

Change-Id: I65b4ad86456c13a3bbc48880e12ab7a034707b88
adb: Add a few more wait-for commands

 * wait-for-recovery is obvious.
 * wait-for-online will wait for recovery || device

Change-Id: I70f747bcd201a4157b988d7ed49bcc8e5ff4c85f
fix up adb to prefer /sbin/sh if it exists

Change-Id: I5c15731c95d123488e90aaeac5b497f556b53363