base.bbclass: When we use fakeroot, also use it for devshell

Its generally useful for devshell to end up in the fakeroot environment. If
a user needs to exit it, PSEUDO_UNLOAD=1 <command> works, its usually
harder to enter the envionment.

[YOCTO #3374]

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