devshell: Don't corrupt the fakeroot variables

The devshell anonymous python fragment overwrites variables in the
datastore with their expanded versions. If this runs before the code
in allarch.bbclass which changes TARGET_OS, we can end up with different
directories in the fakeroot environment variables, some expanded with
the original TARGET_OS value.

The devshell code only needs to run before the task itself so we change
to trigger it to run at task execution time only using a flag.

[YOCTO #4795]

(From OE-Core rev: 56baf177cdf074929a090cc66a8b89d346a5d79c)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/devshell.bbclass

@@ -3,6 +3,15 @@ inherit terminal
 DEVSHELL = "${SHELL}"
 
 python do_devshell () {
+    if d.getVarFlag("do_devshell", "manualfakeroot"):
+       d.prependVar("DEVSHELL", "pseudo ")
+       fakeenv = d.getVar("FAKEROOTENV", True).split()
+       for f in fakeenv:
+            k = f.split("=")
+            d.setVar(k[0], k[1])           
+            d.appendVar("OE_TERMINAL_EXPORTS", " " + k[0])
+       d.delVarFlag("do_devshell", "fakeroot")
+
     oe_terminal(d.getVar('DEVSHELL', True), 'OpenEmbedded Developer Shell', d)
 }
 
@@ -17,11 +26,8 @@ do_devshell[nostamp] = "1"
 # manually
 python () {
     if d.getVarFlag("do_devshell", "fakeroot"):
-       d.prependVar("DEVSHELL", "pseudo ")
-       fakeenv = d.getVar("FAKEROOTENV", True).split()
-       for f in fakeenv:
-            k = f.split("=")
-            d.setVar(k[0], k[1])           
-            d.appendVar("OE_TERMINAL_EXPORTS", " " + k[0])
+       # We need to signal our code that we want fakeroot however we
+       # can't manipulate the environment and variables here yet (see YOCTO #4795)
+       d.setVarFlag("do_devshell", "manualfakeroot", "1")
        d.delVarFlag("do_devshell", "fakeroot")
 }