The NSH shutdown option should be called --reboot, not --reset, for compatibility with the statndard shutdown command

Fix references to the no-longer-existent misc/ directory in comments, README files, and documentation

Documentation:  Add target=_blank to some URLs so they can be opened from DocuWiki.  Some Sourceforge content cannot be served in a frame

Add support for a union file system that can be used to overlay and merge the content of two mounted file systems.

Add basic board build configurat for the SAML21 Xplained.  Initial commit is jsut the SAMD20 Xplained with name changes and does not yet build

Update the type passed to watchdog timer handlers.  Using uint32_t is a problem for 64-bit machines.