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