add scripts/portable_date: wrapper around date for bsd/linux

date -r $unix_timestamp on bsd is equal to date -d @$unix_timestamp on
linux. To support reproducible builds and not loosing every timestamp
it's required to convert a unix timestamp into human readable timestamp

./scripts/portable_date $unix_timstamp +%T

Signed-off-by: Alexander Couzens <lynxis@fe80.eu>

SVN-Revision: 48583