buildroot

reproducibility: override locale and timezone

And export BR2_REPRODUCIBLE for post-build / post-image scripts.

[Peter: Extend commit message,
	move export together with our other exports,
	add comment explaining why we override local/timezone]
Signed-off-by: Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>