cfi: Make the flash erase and write operations abortable

Check for ctrlc() in operations that take time and loop over the flash
addresses.

In netconsole, tstc() is expensive.  Only check once in a while to not
slow down the operation significantly.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Stefan Roese <sr@denx.de>