buildroot

fs: pass EXTRA_ENV to post-fakeroot script

Like we do for post-build and post-image scripts, pass EXTRA_ENV to
post-fakeroot script.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>