devshell: allow the starting directory to be overridden

Some people like to have the devshell start in ${B} instead of ${S}, so allow
this to be overridden.

(From OE-Core rev: 217af639d13258429949b14c67110650288cba7a)

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/devshell.bbclass

@@ -17,7 +17,9 @@ python do_devshell () {
 
 addtask devshell after do_patch
 
-do_devshell[dirs] = "${S}"
+# The directory that the terminal starts in
+DEVSHELL_STARTDIR ?= "${S}"
+do_devshell[dirs] = "${DEVSHELL_STARTDIR}"
 do_devshell[nostamp] = "1"
 
 # devshell and fakeroot/pseudo need careful handling since only the final