system: add selection of a default localtime

[Peter: strip quotes/use local TZDATA_LOCALTIME variable]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[alexandre.belloni: move from "tzdata" to "system configuration"]
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
[yann.morin.1998@free.fr: move into the BR2_TARGET_TZ_INFO conditionnal block]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>