SAMA5D4-EK:  Add documentation/support for Rev E. board. Refresh NSH configuration.  Fix some build errors/warnings introduced with recent work with the knsh configuration

Add the network enabled NSH configuration for the STM32F4Discovery board with the STM32F4DIS-BB base board

Having a symbol table is an option.  There are no symbol tables with CONFIG_BUILD_KERNEL, for example.

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.

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

When a privileged thread exits, we have to use the kernel alloctor to free memory; when an unprivileged thread exits, we don't have to do anything... heap memory will be cleaned up when the address environment is torn down

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

Add logic that will permit us to build user libraries with different CFLAGS than kernel code.  This is needed because we need the -fno-common option when building ELF code to prevent SHN_COMMON relocations

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