If we're not going to change the value of FILE, or we know it isn't
going to have changed (ext == bbclass), don't set FILE.
This avoids messy looking history of the variable as well as optimises
parsing speed slightly.
(Bitbake rev: 88e4600aa66dda2e6c807f9d97af8982bcd8817b)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>