src/flash/startup.tcl: Add preverify to program command

src/flash/startup.tcl: Add preverify to program command

The preverify option allows to check whether flashing is necessary.
If the target is flashed often/automatically this can save time and
preserve the flash. This is expecially helpful in CI environments.

Change-Id: Iead0a269e1a772b751d4dd9e8b53b2fecc874624
Signed-off-by: Moritz 'Morty' Strübe <moritz.struebe@redheads.de>
Reviewed-on: http://openocd.zylin.com/5292
Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>