cmd_sf: add handler for +len arg for erase command

This patch adds [+]len handler for the erase command that will
automatically round up the requested erase length to the flash's
sector_size.

Signed-off-by: Richard Retanubun <RichardRetanubun@RuggedCom.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>