SAM3X/Arduino Due: Fix typo in sam3x_periphclks.h; add SCLK definitions to board.h header file.  From Fabien Comte

SAM3X/Arduino Due: Fix typo in sam3x_periphclks.h; add SCLK definitions to board.h header file.  From Fabien Comte

SAMA5D4-EK: In kernel build with address environment, need logic to map user virtual addresses to physical addresses, and vice versa

Fix some conditional compilation in kernel build mode.  And, while we are touching this logic, make g_default_pthread_attr const.

Remove the 16z board support from the main source tree.  It is still avaialable in the misc/Obsoleted directory

Beginning of logic that will, hopefully, all me to build applications against a NuttX export package (vs. the nuttx/ source tree).