add "adb sideload" and sideload connection state

Recovery will soon support a minimal implementation of adbd which will
do nothing but accept downloads from the "adb sideload" command and
install them.  This is the client side command (mostly resurrected out
of the old circa-2007 "adb recover" command) and the new connection
state.

Change-Id: I4f67b63f1b3b38d28c285d1278d46782679762a2
diff --git a/adb.h b/adb.h
index ac31f11..f7667c2 100644
--- a/adb.h
+++ b/adb.h
@@ -434,6 +434,7 @@
 #define CS_HOST       3
 #define CS_RECOVERY   4
 #define CS_NOPERM     5 /* Insufficient permissions to communicate with the device */
+#define CS_SIDELOAD   6
 
 extern int HOST;
 extern int SHELL_EXIT_NOTIFY_FD;