kinetis: fix "SF1" parts to limit FlexRAM usage

Ensure FlexRAM usage is limited to half the FlexRAM size when programming.
Assume the FlexNVM sector size is equal to half the FlexRAM.
Fix sector erase checking which had an error introduced when the
  kinetis_ftfx_command( ) signature was changed.

Change-Id: I88edd9c7d4a4ba474cad7b00052feaeedfa8ced8
Signed-off-by: Christopher Kilgour <techie@whiterocker.com>
Reviewed-on: http://openocd.zylin.com/1358
Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>