Freedom K64F: Add board_initialize() hooks and bringup C file like more contemporary board configurations

Freedom K64F:  Correct comments about VCOM serial console.  Add pin multi-plexing disambiguaration for the VCOM UART pins.

Add support for the NXP Freedom-K64F board.  This is primarily the work of Jordan Macintyre.  I leveraged this code from https://github.com/jmacintyre/nuttx-k64f

arch/arm/kinetis:  Completes moving register header files to kinetis/chip directory with all K64 changes.

arch/arm/kinetis:  Still moving register header files to kinetis/chip directory; still incorporating K64 differences.

arch/arm/kinetis:  Still moving register header files to kinetis/chip directory; still incorporating K64 differences.

arch/arm/kinetis:  Still moving register header files to kinetis/chip directory; still incorporating K64 differences.

arch/arm/kinetis:  Still moving register header files to kinetis/chip directory; still incorporating K64 differences.

arch/arm/src/kinetis:  Add basic support for the K64 family.  Still moving register definition files to the kinetis/chip subdirectory.

arch/arm/src/kinetis:  Create a chip sub-directory as with other architectures.  Start moving some headers... a lot more still be be moved.

Several MTD FLASH drivers nullify the freed 'priv' structure and failed to return NULL as stated in the comments.  Result, will operate on a NULL pointer later.  Noted by David Sidrane.

Here is a missing register def for STM32L4 after support for dual RX. Thanks to Paul for porting the CAN improvements to both STM32 targets.

stm32_can

Shifting 1 by 31 is undefined behaviour because 1 is signed.
We should probably use 1ul instead of 1 everywhere else,
but for now this silences a compiler warning.