devshell: Don't corrupt the fakeroot variables

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>