Flash driver improvements.

- Moved the Kinetis security check function to a new Flash_Kinetis base
  class, from which all Kinetis flash algos now derive.
- Improved security check to disallow an FOPT byte of 0x00.
- Removed specific Kinetis L-series flash algo classes in favour of a
  new Flash_klxx class, since the flash blob is the same for all current
  L-series devices.
- Changed Flash.programPage() to pass the length of the data array
  instead of a fixed page size, to allow to programming less than a full
  page, if the device supports it.

Signed-off-by: Chris Reed <flit@me.com>