SAML21 clock config: Fix a misthink in last commit.  Move setting of ONDEMAND to after clock is enabled in most cases

SAML21: Add logic to support GLCK peripheral channels; add logic to configure FDPLL lock timer and ref clock GCLKs

SAML21-Xplained:  Update the cloned SAMD20-Xplained board configuration for specifics of the SAML21-Xplained

SAMD/L: Hide differences in the way peripheral clocking is enabled behind macros.  Some updates to saml_clockconfig.h + various fixes while working toward SAML21 build

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

SAMD/L: Move content of chip/sam_memorymap.h to chip.h; move chip/sam_pinmap. to sam_pinmap.h for consistency with other header files

Rename SAMD directories and configuration variables to SAMDL so that the same build environment can support the SAML

SAMV71-XULT: Reads from LCD memory do not work.  Change configuration so that the LCD is treated as write-only