Rename everything associated with the dynamic process stack to ustack to make room in the name space for a kstack

Move static helper routines from arm_addrenv.c and may them global so that they can be shared both forthcoming stack address environment logic.

ARMv7-A: Modify up_fullcontextrestore() for CONFIG_BUILD_KERNEL.  It changed CPSR while in kernel.  That will crash is the new CPSR is user mode while executing in kernel space.  Fixed by adding a SYS_context_restore system call.  There is an alternative, simpler modification to up_fullcontextrestore() that could have been done:  It might have been possible to use the SPSR instead of the CPRSR and then do an exception return from up_fullcontextrestore().  That would be more efficient, but I never tried it.

All tasks, even user mode tasks, must start in supervisor mode until they get past the start-up trampoline

ARMv7-A: Exception register save/restore needs to work a little differently if we support user mode processes

Misc fixes to repair some of the breakage to the SAMA5D4-EK elf configuration caused by changes for the knsh configuration

SAMA5D4-EK: These configurations now use the fixed DRAM mapping for manipulating the page memory pool.

Add configuration to use the fixed DRAM mapping for the page pool (if available) instead of remapping dynamically to access L2 page tables and page data.  Also, add logic in address environment creation to initialize the shared data at the beginning of the .bss/.data process memory region.

ELF relocations.  Some relocation types do not have a named symbol associated with them.  The design did not account for that case

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

The 'make export' target needs to bundle up the user C startup file (crt0), not the kernel head object

Space at the beginning of the process data space is now reserved for user heap management structures.  In the kernel build mode, these heap structures are shared between the kernel and use code in order to allocate user-specific data.

There used to be two ways to pass parameters to new tasks, depending upon the configuration:  Either (1) argv[] as created as an array with each string strdup'ed.  Or (1) argv[] array and strings were created on the stack before the new task was started.   Now, there is only one way, way (1).  Way (2) might be slightly more compact, but this is not worth carry the complexity of two different ways of doing the same thing.

Obsolete support for the 8051 family.  Trying to maintain compatibility with this family is too much effort and there has never really been a successful NuttX impplementation on these parts