Print three-letter abbreviations of the days and months.

With a fixup by Andrey Smirnov:
| common/date.c: Fix off-by-one error
|
| As per http://pubs.opengroup.org/onlinepubs/007908775/xsh/time.h.html
| 'tm_wday' is zero indexed with zero representing Sunday, this is also
| corroborated by the code in rtc_time_to_tm() which used 4 to represent
| Thursday.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>