system: Defaulting TZ_LOCALTIME to UTC

This patch sets the default timezone to UTC if not overwritten.
Some packages need a configured system timezone for properly
operating like mono based software.

Signed-off-by: Angelo Compagnucci <angelo.compagnucci@gmail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>