EEPROM support: handle implicit erase

Add support for EEPROMs with 1 byte granularity and implicit erase on
write. flashrom will not try to erase before write on these chips.
Explicitly requested erase with -E is done by writing 0xff.

Signed-off-by: Carl-Daniel Hailfinger <c-d.hailfinger.devel.2006@gmx.net>
Acked-by: Stefan Tauner <stefan.tauner@alumni.tuwien.ac.at>


git-svn-id: svn://flashrom.org/flashrom/trunk@1822 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1