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>

system/Config.in

@@ -369,6 +369,7 @@ config BR2_TARGET_TZ_ZONELIST
 
 config BR2_TARGET_LOCALTIME
 	string "default local time"
+	default "Etc/UTC"
 	help
 	  The time zone to install as the default local time, expressed as a
 	  tzdata location, such as: