Remove two perl calls with sed, and adds an alternative

Remove two perl calls with sed, and adds an alternative
for another two (using GNU date, and with limited impact if both
alternatives fail)

Those were the remaining perl calls in our build tree, so remove perl
from our dependency list in README.

Signed-off-by: Patrick Georgi <patrick.georgi@coresystems.de>
Acked-by: Stefan Reinauer <stepan@coresystems.de>
git-svn-id: svn://svn.coreboot.org/coreboot/trunk@5108 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1