timestamp-service: use new format also for writing

https://github.com/Angstrom-distribution/meta-angstrom/commit/8228b2d56ef2415c8b96d1463fb38816279d4c74
changes the time format written to /etc/timestamp
When the system time is set from the content of /etc/timestamp,
one has to use the new format also.

While at it
- remove the now unneeded cut and concatenation as the string is already
  correctly ordered.
- consistently use absolute pathnames to programs
- don't print warnings when /etc/timestamp does not exist

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>