Optional canned symtab inclusion to the build. When option CONFIG_LIBC_SYMTAB is selected and symbol table file libc/symtab/canned_symtab.inc is prepared then application can use system provided complete symbol table. The option has substantial effect on system image size. Mainly code/text. If loading of applications at runtime is not planned do not select this.  From Pavel Pisa.

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