Modify the default path of dropbear to include /sbin and /usr/sbin.

Index: buildroot/package/dropbear/0001-defaultpath.patch
===================================================================
--- /dev/null	1969-12-31 19:00:00.000000000 -0500
+++ buildroot/package/dropbear/0001-defaultpath.patch	2017-10-02 13:32:32.197369170 -0400
@@ -0,0 +1,13 @@
+diff -Naur a/options.h b/options.h
+--- a/options.h	2017-10-02 10:40:56.591817649 -0400
++++ b/options.h	2017-10-02 13:31:45.493262641 -0400
+@@ -342,7 +342,7 @@
+ #define DEFAULT_IDLE_TIMEOUT 0
+ 
+ /* The default path. This will often get replaced by the shell */
+-#define DEFAULT_PATH "/usr/bin:/bin"
++#define DEFAULT_PATH "/usr/bin:/bin:/usr/sbin:/sbin"
+ 
+ /* Some other defines (that mostly should be left alone) are defined
+  * in sysoptions.h */
+