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>
(cherry picked from commit 7776d287c3a6666d3789ac0fc706a12b079c1dc7)
Signed-off-by: Max Krummenacher <max.oss.09@gmail.com>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>
timestamp-service: don't restore time on shutdown

load-timestamp.sh --save should only save the current systemtime and not
evaluate /etc/timestamp and change the systemtime.
If the systemtime has been changed to something in the past of /etc/timestamp
then that change should be accepted without further checks.

While at it correct the comment. The RTC/hwclock is not touched by the
timestamp service. This is a copy paste error from the init.d implementation
in openembedded-core/meta/recipes-core/initscripts/initscripts-1.0/bootmisc.sh

Signed-off-by: Max Krummenacher <max.oss.09@gmail.com>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>