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.

Add logic to support custom board directories that include a Kconfig file.  During the context phase of the build, any Kconfig file in the custom board directory is copied into configs/dummy, replacing the existing Kconfig file with the target Kconfig file.

 - Bugfix: stdfilters didn't work because the filter was never enabled (wrong number of bits to shift)
 - Bugfix: Filters were never used because the configuration register cannot be written without using the initialization mode

Both bugs are fixed by this patch. Filtering has been tested with both standard and extended identifiers and is now working properly.

Upstream_nucleo 144